Master Orthopedic Lower Extremity Biomechanics, Gait & Deformity Correction for ABOS Board Review | Part 7
Comprehensive 100-Question Exam
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Question 1
When planning a corrective osteotomy for a tibial diaphyseal deformity, an opening wedge osteotomy is designed at a level distal to the center of rotation of angulation (CORA). To restore the mechanical axis without creating a secondary translational deformity, what must be incorporated into the surgical plan?
Explanation
According to Paley's osteotomy rules, if the osteotomy and hinge are at a level different from the CORA, translation must be incorporated to properly align the proximal and distal mechanical axes. Failure to translate will result in a secondary mechanical axis deviation.
Question 2
During normal human walking, at which phase of the gait cycle does the gluteus maximus exhibit its peak muscle activity?
Explanation
The gluteus maximus acts eccentrically during initial contact and loading response to decelerate the forward momentum of the trunk and prevent excessive hip flexion. This phase represents its peak activity during the gait cycle.
Question 3
According to the principles of deformity correction (Paley's rules), if an osteotomy is performed at a level distinct from the center of rotation of angulation (CORA), but the hinge axis is placed exactly on the CORA, what is the geometric result?
Explanation
According to Osteotomy Rule 2, when the hinge is placed on the CORA but the osteotomy is at a different level, the mechanical axis will be realigned, but the bone ends will translate at the osteotomy site. This is frequently utilized when the CORA is in an undesirable location for bone healing.
Question 4
In a patient presenting with an isolated paresis of the tibialis anterior, which of the following gait abnormalities is most likely to be observed during the loading response phase?
Explanation
During the loading response phase, the tibialis anterior normally contracts eccentrically to lower the foot to the ground smoothly. Weakness results in an inability to control this descent, leading to an audible 'foot slap'.
Question 5
A patient with osteoarthritis presents with a varus knee deformity. Standing full-length radiographic analysis reveals a mechanical lateral distal femoral angle (mLDFA) of 88 degrees and a mechanical medial proximal tibial angle (mMPTA) of 78 degrees. What is the primary source of the mechanical axis deviation?
Explanation
The normal mLDFA is 85-90 degrees (average 88) and normal mMPTA is 85-90 degrees (average 87). An mMPTA of 78 degrees is abnormally low, unequivocally indicating a proximal tibia varus deformity as the primary driver of the alignment anomaly.
Question 6
A patient presents with a severe limp secondary to hip osteoarthritis. Gait analysis reveals that during the stance phase on the right leg, the patient significantly leans their trunk laterally over the right hip. Which biomechanical alteration primarily explains the benefit of this compensatory mechanism?
Explanation
A lateral trunk lean over the affected hip (compensated Trendelenburg gait) shifts the body's center of gravity closer to the hip joint's center of rotation. This shortens the lever arm of body weight, drastically reducing the force required by the hip abductors and lowering the overall hip joint reaction force.
Question 7
When optimizing the biomechanical stability of an Ilizarov circular external fixator, which of the following frame modifications will most effectively increase the bending stiffness of the construct?
Explanation
To increase frame stiffness in an Ilizarov construct, one should decrease ring diameter, increase wire tension, cross wires as close to 90 degrees as possible, and decrease the distance between the rings closest to the fracture or osteotomy site.
Question 8
During the terminal stance phase of normal gait, what is the anatomical relationship of the ground reaction force (GRF) vector relative to the major joints of the lower extremity?
Explanation
In terminal stance, the GRF vector is located anterior to the ankle (producing a dorsiflexion moment), anterior to the knee (producing an extension moment), and posterior to the hip (producing an extension moment).
Question 9
At what phases of the normal gait cycle does the vertical ground reaction force reach its two primary peaks?
Explanation
During a normal gait cycle, the vertical ground reaction force is bimodal. It peaks at approximately 120% of body weight during loading response (weight acceptance) and again during terminal stance (push-off).
Question 10
According to Paley's rules of deformity correction, an opening wedge osteotomy performed exactly at the Center of Rotation of Angulation (CORA) will result in:
Explanation
Osteotomy Rule 1 states that if the osteotomy and the mechanical axis hinge are both placed at the CORA, the angulation will be fully corrected with no translation of the bone segments.
Question 11
A patient with right hip osteoarthritis is advised to use a cane. Holding the cane in the contralateral (left) hand reduces the right hip joint reaction force primarily by:
Explanation
Using a cane in the contralateral hand provides a long lever arm that generates an upward moment. This counteracts the moment of body weight, drastically reducing the force required by the ipsilateral hip abductors and lowering the total joint reaction force.
Question 12
In a child with spastic diplegic cerebral palsy, true crouch gait is typically characterized by which combination of lower extremity joint positions during stance phase?
Explanation
True crouch gait is defined by excessive hip flexion, knee flexion, and ankle dorsiflexion (calcaneus) during the stance phase. It is often caused by over-lengthened Achilles tendons in the presence of spastic/contracted hamstrings and hip flexors.
Question 13
Which of the following modifications most effectively increases the axial stiffness of an Ilizarov circular ring fixator construct?
Explanation
Frame stability is maximized by using smaller diameter rings (closer to the limb), placing rings closer to the fracture/osteotomy site, increasing wire tension, and using wires that cross at 90-degree angles.
Question 14
A patient with severe bilateral coxa valga presents for biomechanical evaluation. How does coxa valga structurally alter hip joint biomechanics compared to normal anatomy?
Explanation
Coxa valga is associated with a more vertical femoral neck, which decreases the lever arm of the hip abductors. This forces the abductors to contract more forcefully to maintain a level pelvis, significantly increasing the hip joint reaction force.
Question 15
During normal gait, "foot slap" immediately following initial contact is clinically prevented by which of the following muscular actions?
Explanation
From initial contact to loading response, the ankle rapidly plantarflexes. The tibialis anterior and other ankle dorsiflexors contract eccentrically to control this descent, preventing an abrupt "foot slap."
Question 16
When performing a medial opening wedge high tibial osteotomy (HTO) for a varus deformity, what is the most common unintended sagittal plane alteration if the osteotomy gap is opened symmetrically anteriorly and posteriorly?
Explanation
Because the medial tibia is triangular (narrower anteriorly than posteriorly), a symmetric rectangular opening wedge will disproportionately prop open the anterior aspect. This commonly results in an unintended increase in the posterior tibial slope.
Question 17
A 10-year-old child undergoes guided growth with a medial tension band plate for genu valgum. The implant exerts its corrective effect primarily through which of the following biological principles?
Explanation
The Hueter-Volkmann principle states that increased compression across an active physis slows longitudinal growth, whereas decreased compression accelerates it. A medial tension band plate restricts medial physeal growth while allowing lateral growth to continue, correcting the valgus.
Question 18
Which of the following classical "determinants of gait" primarily functions to limit the maximal upward vertical excursion (peak) of the body's center of mass during the mid-stance phase?
Explanation
Pelvic tilt, or dropping of the pelvis on the non-weight-bearing (swing) side by about 5 degrees, effectively flattens the gait arc. This lowers the peak vertical elevation of the center of mass during mid-stance.
Question 19
Evaluating a lower extremity deformity, the mechanical axis deviation (MAD) falls deep into the lateral compartment of the knee. The mechanical lateral distal femoral angle (mLDFA) is 75 degrees and the medial proximal tibial angle (MPTA) is 88 degrees. What is the primary source of the deformity?
Explanation
The normal mLDFA is approximately 88 degrees. An mLDFA of 75 degrees indicates a smaller than normal angle, meaning the distal femur is structurally in severe valgus. The normal MPTA confirms the tibia is not contributing to the primary angular deformity.
Question 20
According to Paley's rules of deformity correction, if an osteotomy is performed at a level different from the Center of Rotation of Angulation (CORA), but the mechanical hinge is placed exactly on the CORA, what will be the resulting geometric alignment?
Explanation
Osteotomy Rule 2 states that placing the axis of rotation at the CORA but making the osteotomy cut at a different level will completely correct the angular deformity. However, collinearity is restored at the cost of a planned, expected translation of the bone segments at the osteotomy site.
Question 21
During normal level walking, at what phase of the gait cycle does the compressive joint reaction force at the ankle reach its maximum, and what is its approximate magnitude relative to body weight (BW)?
Explanation
The ankle joint reaction force peaks during terminal stance (heel off) due to the massive contraction of the triceps surae required for push-off. This compressive force routinely reaches 4 to 5 times body weight (and sometimes higher).
Question 22
A patient exhibits a compensated Trendelenburg gait on the right side. During the stance phase of the right leg, which of the following kinematic adaptations is predominantly observed?
Explanation
In a compensated Trendelenburg gait, the patient leans their trunk laterally over the weak stance hip (right side). This shifts the body's center of mass closer to the hip joint center, reducing the moment arm of body weight and decreasing the demand on the weak abductor muscles.
Question 23
The Taylor Spatial Frame (TSF) utilizes Stewart-Gough platform biomechanics. How many independent parameters (degrees of freedom) must be established to fully define the spatial deformity of the moving fragment relative to the reference fragment?
Explanation
The Taylor Spatial Frame corrects deformities in all six degrees of freedom: anterior-posterior translation, lateral-medial translation, axial translation (length), anterior-posterior angulation (procurvatum/recurvatum), lateral-medial angulation (varus/valgus), and axial rotation.
Question 24
When establishing reference lines for complex tibial deformity planning, the mechanical axis of the normal tibia is defined as a line connecting the center of the ankle plafond to which of the following proximal landmarks?
Explanation
The mechanical axis of the tibia is defined by a straight line connecting the center of the proximal tibial plateau (often just medial to the lateral tibial spine) to the center of the distal tibial plafond.
Question 25
The "Windlass mechanism" of the foot, which is essential for normal rigid lever biomechanics during terminal stance, is primarily activated by which of the following kinematic actions?
Explanation
Hicks' windlass mechanism is engaged by dorsiflexion of the first metatarsophalangeal (MTP) joint during late stance. This action winds the plantar fascia around the metatarsal heads, elevating the longitudinal arch and locking the midfoot for efficient push-off.
Question 26
A patient with a 4 cm structural true limb length discrepancy (short right leg) presents for gait evaluation. Which of the following is the most common biomechanical compensation observed during the stance phase of the short (right) extremity?
Explanation
To functionally lengthen a short limb and reach the ground without dropping the pelvis excessively, patients typically adopt an equinus (plantarflexed) posture at the ankle on the shorter side during stance.
Question 27
According to Paley's rules of deformity correction, if an osteotomy is performed at a level different from the Center of Rotation of Angulation (CORA), but the axis of correction (hinge) is placed exactly at the CORA, what is the resulting effect on the bone?
Explanation
Paley's Rule 2 states that if the osteotomy is distinct from the CORA but the hinge (axis of correction) is at the CORA, angulation and translation occur simultaneously. This results in perfectly realigned (collinear) mechanical axes.
Question 28
During the normal human gait cycle, which of the following muscles acts eccentrically during the loading response (first rocker) to control the deceleration of the foot to the ground?
Explanation
The tibialis anterior acts eccentrically from initial contact to foot flat (loading response) to prevent foot slap. The gastrocnemius and soleus act eccentrically later in stance (second rocker) to control tibial advancement.
Question 29
A patient uses a cane in the contralateral hand to relieve pain from unilateral hip osteoarthritis. By what biomechanical mechanism does this intervention primarily decrease the hip joint reaction force on the affected side?
Explanation
Using a cane in the contralateral hand creates an upward ground reaction force with a long moment arm to the affected hip. This generates a counter-moment that significantly reduces the force required by the affected hip abductors, thereby exponentially decreasing the total hip joint reaction force.
Question 30
A patient presents with a 'Trendelenburg gait' characterized by a lateral trunk lean towards the stance limb. Which of the following best describes the biomechanical rationale for this compensatory trunk shift?
Explanation
In a Trendelenburg gait, shifting the trunk laterally over the weak stance hip moves the body's center of gravity closer to the hip joint center. This reduces the moment arm of body weight, drastically decreasing the demand on the weak gluteus medius to maintain pelvic leveling.
Question 31
When performing a medial opening wedge high tibial osteotomy (HTO), which of the following intraoperative technical errors is most likely to inadvertently increase the posterior tibial slope?
Explanation
The proximal tibia has a naturally triangular shape. Opening the anterior cortex equally to the posterior cortex in a medial opening wedge HTO will inadvertently increase the posterior tibial slope; typically, the anterior gap must be roughly half the posterior gap to maintain slope.
Question 32
In normal gait, what is the peak amount of knee flexion achieved during the swing phase to ensure adequate foot clearance?
Explanation
Normal human gait requires approximately 60 to 65 degrees of knee flexion during the initial and mid-swing phases to ensure adequate foot clearance. The knee then extends again in preparation for initial contact.
Question 33
A patient has a severe distal femoral valgus deformity. On a standing full-length anteroposterior radiograph, how is the Mechanical Axis Deviation (MAD) accurately defined?
Explanation
Mechanical Axis Deviation (MAD) is measured in millimeters as the perpendicular distance from the center of the knee joint to the mechanical axis line (connecting the center of the femoral head to the center of the ankle talus).
Question 34
During a total hip arthroplasty, the surgeon decides to use a femoral stem with increased lateral offset. What is the primary biomechanical consequence of this adjustment?
Explanation
Increasing femoral offset extends the distance from the center of rotation to the line of action of the abductor muscles. This increased moment arm improves abductor mechanical advantage, thereby reducing the required muscle force and decreasing overall joint reaction forces.
Question 35
When applying a Taylor Spatial Frame for the correction of a multiplanar tibial deformity, what is the most critical function of the 'mounting parameters' entered into the software?
Explanation
In the Taylor Spatial Frame system, mounting parameters strictly define the position and orientation of the reference ring to the underlying bone segment. Accuracy here is absolutely critical, as the software algorithm depends on this relationship to calculate the exact spatial deformity correction path.
Question 36
During tibial lengthening over an intramedullary nail (LON) for limb length discrepancy, a patient develops insidious onset of clawing of the great toe and decreased sensation in the first web space. What is the most appropriate initial management step?
Explanation
The patient is presenting with signs of deep peroneal nerve stretch injury, a known complication of aggressive tibial lengthening. The most appropriate initial step is to halt the lengthening process; occasionally, backing up the distractor may be required to relieve tension on the nerve.
Question 37
Which of the following descriptions best characterizes the 'second rocker' of the normal gait cycle?
Explanation
The second rocker (ankle rocker) occurs during mid-stance when the tibia rolls forward over the planted foot. This forward momentum is eccentrically controlled by the soleus and gastrocnemius muscles to prevent premature dorsiflexion collapse.
Question 38
According to Paley's deformity correction principles, what occurs if both the osteotomy and the axis of correction (hinge) are located outside the Center of Rotation of Angulation (CORA)?
Explanation
Paley's Rule 3 dictates that if the osteotomy and hinge are placed away from the CORA, the mechanical axes of the proximal and distal segments will become parallel but not collinear. This effectively introduces a new translation deformity.
Question 39
A 16-year-old female presents with bilateral in-toeing. Clinical examination reveals 80 degrees of hip internal rotation and 10 degrees of hip external rotation bilaterally. Which of the following compensatory gait abnormalities is most likely to be observed simultaneously?
Explanation
The clinical examination indicates excessive femoral anteversion. These patients often develop compensatory external tibial torsion to keep the feet relatively straight during gait, leading to a complex torsional profile known as Miserable Malalignment Syndrome.
Question 40
Which of the following mechanical environments is MOST critical to optimize distraction osteogenesis during the lengthening phase using a circular external fixator?
Explanation
Distraction osteogenesis thrives on controlled axial micromotion (dynamization), which stimulates bone formation and maturation. However, shear and torsional forces are highly detrimental to the regenerate and must be minimized by the tensioned wire circular frame construct.
Question 41
An opening wedge osteotomy is planned for a distal femoral valgus deformity.
The osteotomy cut is made through the Center of Rotation of Angulation (CORA), and the hinge is placed on the convex cortex exactly at the CORA. Which osteotomy rule is being applied, and what is the expected outcome regarding translation?
The osteotomy cut is made through the Center of Rotation of Angulation (CORA), and the hinge is placed on the convex cortex exactly at the CORA. Which osteotomy rule is being applied, and what is the expected outcome regarding translation?
Explanation
Under Osteotomy Rule 1, both the osteotomy cut and the hinge are located at the CORA. This results in pure angulation and realignment of the mechanical axis without any translation of the bone fragments.
Question 42
During the stance phase of gait, a patient exhibits a contralateral pelvic drop accompanied by a compensatory lateral trunk lean toward the stance limb. Biomechanically, what is the primary purpose of this lateral trunk lean?
Explanation
A Trendelenburg gait with a lateral trunk lean shifts the body's center of gravity closer to the center of rotation of the hip joint. This reduces the lever arm of the body weight, thereby decreasing the torque that the weakened hip abductors must counteract.
Question 43
A patient presents with a severe varus deformity of the lower extremity. Mechanical Axis Deviation (MAD) is measured on a standing long-leg alignment radiograph. How is the MAD correctly defined?
Explanation
Mechanical Axis Deviation (MAD) is defined as the perpendicular distance from the center of the knee joint to the mechanical axis of the lower extremity. In a normally aligned limb, the mechanical axis passes slightly medial to or through the exact center of the knee.
Question 44
During the mid-stance phase of normal gait, where does the Ground Reaction Force (GRF) vector typically pass in the sagittal plane relative to the hip, knee, and ankle joint centers?
Explanation
At mid-stance, the GRF vector passes posterior to the hip (creating an extension moment), anterior to the knee (creating an extension moment), and anterior to the ankle (creating a dorsiflexion moment).
Question 45
In a patient with a complex triplanar proximal tibial deformity (varus, procurvatum, and internal rotation), a single-cut focal dome osteotomy is planned. According to the principles of multiplanar deformity correction, how must the axis of rotation for the osteotomy be oriented?
Explanation
To correct a multiplanar deformity with a single osteotomy, the deformity must be calculated as a single oblique plane deformity. The axis of correction (hinge) must be placed perpendicular to this plane of maximal deformity.
Question 46
When utilizing a hexapod external fixator (e.g., Taylor Spatial Frame) for lower extremity deformity correction, what is the primary consequence of inaccurately defining the 'mounting parameters'?
Explanation
Mounting parameters define the position of the reference ring relative to the reference bone fragment. If these are incorrect, the software calculates rotation and translation around an incorrect virtual hinge, inducing iatrogenic deformities.
Question 47
According to the classic Ilizarov principles of distraction osteogenesis, what are the optimal latency period, distraction rate, and rhythm for a standard adult tibial diaphyseal lengthening?
Explanation
Ilizarov established that optimal regenerate bone formation in adults occurs with a latency phase of 7-10 days followed by a distraction rate of 1 mm per day, divided into a rhythm of 0.25 mm every 6 hours.
Question 48
During a medial opening wedge high tibial osteotomy (HTO) for a varus knee, the anterior gap is inadvertently opened significantly more than the posterior gap. What is the expected biomechanical effect on the knee?
Explanation
Opening the anterior cortex more than the posterior cortex increases the posterior tibial slope. Biomechanically, an increased posterior slope shifts the tibia anteriorly under axial load, placing increased strain on the anterior cruciate ligament (ACL).
Question 49
A patient with patella alta complains of anterior knee pain and weakness during stair climbing. How does patella alta biomechanically alter the extensor mechanism during early knee flexion?
Explanation
Patella alta delays engagement of the patella into the trochlear groove during early flexion. This effectively decreases the moment arm of the extensor mechanism, requiring higher quadriceps force to maintain torque, which can lead to anterior knee pain.
Question 50
The 'Windlass mechanism' is critical for arch support during normal gait. Which phase of the gait cycle relies most heavily on this mechanism, and which anatomical structure is primarily responsible for its effect?
Explanation
The Windlass mechanism occurs during terminal stance and pre-swing when the metatarsophalangeal joints dorsiflex. This winds the plantar fascia around the metatarsal heads, elevating and stabilizing the longitudinal arch for push-off.
Question 51
A patient with severe right hip osteoarthritis is advised to use a cane. To maximally decrease the joint reactive forces across the affected hip, how should the cane be used, and what is the primary biomechanical mechanism?
Explanation
Using a cane in the contralateral hand provides an upward force at a long distance from the affected hip. This generates a counter-moment that significantly reduces the force required by the hip abductors to maintain pelvic level, substantially lowering the total joint reactive force.
Question 52
During the loading response phase of a normal gait cycle, where does the ground reaction force (GRF) vector pass relative to the knee joint, and which muscle group is primarily active to control this motion?
Explanation
In the loading response, the GRF vector passes posterior to the knee, creating a flexion moment. The quadriceps fire eccentrically to absorb shock and control this early knee flexion.
Question 53
According to Paley's rules of deformity correction (Rule 3), if an osteotomy is performed at a level separate from the Center of Rotation of Angulation (CORA) and the hinge axis is placed exactly at the osteotomy site, what is the resulting geometric change to the mechanical axis?
Explanation
Paley's Rule 3 states that if the osteotomy and hinge are both placed away from the CORA, angular correction will be accompanied by an iatrogenic translational deformity. To avoid this, the hinge must be kept on the CORA.
Question 54
In an uncompensated Trendelenburg gait due to right gluteus medius weakness, which kinematic abnormality is observed during the stance phase of the right lower extremity?
Explanation
In an uncompensated Trendelenburg gait, weak abductors on the stance leg (right) fail to maintain a level pelvis, causing the pelvis to drop on the contralateral (left) swing side.
Question 55
During the terminal stance phase of normal human walking, what is the primary role of the triceps surae (gastroc-soleus complex)?
Explanation
During terminal stance, the gastroc-soleus complex primarily fires eccentrically to control the forward advancement of the tibia over the foot (controlling dorsiflexion). Peak concentric push-off power occurs slightly later in pre-swing.
Question 56
A 6-year-old child with spastic diplegic cerebral palsy presents with a severe crouch gait pattern. This pattern is primarily characterized by excessive knee flexion during stance. Which of the following secondary pelvic deviations is most typically associated with crouch gait due to hamstring tightness?
Explanation
Crouch gait involves excessive hip and knee flexion. When spastic, shortened hamstrings are the primary driver, they exert a downward pull on the ischial tuberosities, leading to a compensatory posterior pelvic tilt.
Question 57
When analyzing the mechanical axis of the lower extremity for a complex femoral deformity
, the mechanical axis of the femur is normally oriented at what angle relative to the anatomic axis of the femur?
, the mechanical axis of the femur is normally oriented at what angle relative to the anatomic axis of the femur?
Explanation
The normal anatomic axis of the femur is typically in 5 to 7 degrees of valgus relative to its mechanical axis (which connects the center of the femoral head to the center of the knee).
Question 58
Patellofemoral joint reaction forces (PFJRF) vary significantly depending on the activity. Among the following activities of daily living, which generates the highest peak PFJRF?
Explanation
Descending stairs generates the highest patellofemoral joint reaction forces, often reaching up to 7 to 8 times body weight due to the high eccentric quadriceps demand required to lower the body's center of gravity.
Question 59
A patient with an absent tibialis anterior muscle will exhibit a specific gait abnormality during the initial contact to loading response phase. Which of the following best describes this compensatory gait pattern?
Explanation
The tibialis anterior typically acts eccentrically from initial contact to loading response to gently lower the foot to the ground. Absence of this eccentric control results in a 'foot slap'.
Question 60
A 10-year-old girl is evaluated for a leg length discrepancy (LLD). According to standard growth charts and multiplier methods, at approximately what age do normal girls achieve 50% of their mature lower extremity length?
Explanation
Girls typically reach 50% of their final mature lower extremity length at approximately 3 years of age, whereas boys reach this milestone at approximately 4 years of age.
Question 61
In the principles of distraction osteogenesis (Ilizarov technique)
, what is the optimal rate and rhythm for distraction to promote robust bone regenerate without causing premature consolidation or nonunion?
, what is the optimal rate and rhythm for distraction to promote robust bone regenerate without causing premature consolidation or nonunion?
Explanation
The classic Ilizarov principle of tension-stress dictates an optimal distraction rate of 1.0 mm per day, divided into a high-frequency rhythm of 0.25 mm four times a day, to optimize bone regeneration and soft tissue adaptation.
Question 62
A surgeon performs a total hip arthroplasty and intentionally decreases the femoral offset compared to the patient's native anatomy. What is the most significant biomechanical consequence of this change?
Explanation
Decreasing femoral offset shortens the abductor moment arm. To maintain pelvic equilibrium against body weight, the abductor muscles must generate significantly more force, which concurrently increases the overall joint reactive force.
Question 63
The 'screw-home' mechanism of the knee is essential for locking the joint in terminal extension. During the final 20 degrees of open-chain knee extension, what specific kinematic motion occurs?
Explanation
In open-chain knee extension, the tibia externally rotates relative to the femur during the final degrees of extension to 'lock' the knee. In closed-chain extension, the femur internally rotates on the fixed tibia.
Question 64
Human gait utilizes several determinants to minimize the displacement of the center of gravity (COG), thereby reducing energy expenditure. Which of the following mechanisms is most responsible for limiting the highest vertical excursion of the COG during midstance?
Explanation
Knee flexion during the stance phase (approximately 15 degrees) shortens the lower extremity during midstance, effectively limiting the peak vertical excursion of the center of gravity.
Question 65
To accurately locate the center of rotation of angulation (CORA) in a uniapical diaphyseal deformity, a surgeon must plot two specific lines. Which of the following defines the CORA?
Explanation
The CORA is defined geometrically as the point where the proximal mechanical (or anatomical) axis intersects the distal mechanical (or anatomical) axis of the deformed bone segment.
Question 66
A patient is evaluated for a varus knee deformity. Radiographs reveal a Joint Line Convergence Angle (JLCA) of 6 degrees (normal is 0-2 degrees). This finding most strongly indicates the presence of which of the following?
Explanation
An abnormally widened Joint Line Convergence Angle (JLCA) indicates that the joint lines of the femur and tibia are not parallel. This points to an intra-articular deformity, such as asymmetric cartilage loss or collateral ligament laxity.
Question 67
A 45-year-old male demonstrates a high steppage gait, characterized by excessive hip and knee flexion during the swing phase. This kinematic deviation is primarily an obligatory compensation for which underlying deficit?
Explanation
A high steppage gait is a compensation for weak ankle dorsiflexors (foot drop). The patient excessively flexes the hip and knee to clear the plantarflexed foot off the ground during swing phase.
Question 68
In biomechanical planning for fracture fixation, a surgeon contemplates replacing a stainless steel compression plate with a titanium plate of identical geometric dimensions. How will the bending stiffness of the construct change, and which material property is responsible?
Explanation
Bending stiffness depends on both geometry (area moment of inertia) and the material's elastic modulus (Young's modulus). Titanium has a lower elastic modulus than stainless steel, making a plate of identical dimensions less stiff.
Question 69
The Taylor Spatial Frame (TSF) utilizes the Stewart-Gough platform principle for complex deformity correction. Through its six adjustable struts, the TSF is mathematically capable of simultaneous correction in a maximum of how many degrees of freedom?
Explanation
The hexapod system controls spatial position via 6 degrees of freedom: translation in three planes (coronal, sagittal, axial) and rotation in three planes (angulation in coronal/sagittal planes and rotation in the axial plane).
Question 70
According to Paley's Rule 1 of deformity correction, when the osteotomy is performed exactly at the CORA and the hinge axis is also placed exactly at the CORA, what is the kinematic outcome of the opening wedge correction?
Explanation
Paley's Rule 1 states that if both the osteotomy and the hinge of the corrective device are located at the CORA, the mechanical and anatomical axes will correct collinearly with pure angulation and no secondary translation.
Question 71
Peak knee flexion during the stance phase of normal human walking occurs at which specific event, and what is its primary function?
Explanation
Peak knee flexion during the stance phase (approximately 15 to 20 degrees) occurs at the end of the loading response (about 15% of the gait cycle). Its primary biomechanical function is to absorb the shock of initial ground contact.
Question 72
According to Paley's rules of deformity correction, if an osteotomy is performed at a level outside the Center of Rotation of Angulation (CORA) and the hinge is placed at the osteotomy site, what is the geometric result of the correction?
Explanation
Paley's Rule 3 dictates that when both the osteotomy and the hinge are located at a level distinct from the CORA, correcting the angular deformity will inadvertently cause the mechanical axis to translate, creating a secondary translational deformity.
Question 73
In a static single-leg stance, the abductor muscle force required to maintain a level pelvis is primarily determined by the ratio of the body weight lever arm to the abductor lever arm. What is the approximate ratio of these lever arms (body weight : abductors) in a normal adult pelvis?
Explanation
The lever arm for body weight from the center of the femoral head is roughly three times longer than the lever arm for the abductor muscles. Consequently, the abductors must exert a force approximately three times body weight to maintain a level pelvis.
Question 74
During normal human walking, at which phase of the gait cycle does the ankle plantarflexor muscle group generate its maximum concentric power (often referred to as the A2 power burst)?
Explanation
The peak concentric power generation of the ankle plantarflexors occurs during terminal stance and transitions into pre-swing. This provides the primary propulsive force (push-off) for forward progression of the body.
Question 75
When utilizing a hexapod external fixator (e.g., Taylor Spatial Frame) for multiplanar deformity correction, the user must input accurate "mounting parameters" into the software. What do these parameters specifically define?
Explanation
Mounting parameters define the exact spatial position (anteroposterior, lateral, and axial translation) of the reference ring relative to the bone it is fixed to. Inaccurate mounting parameters will result in an erroneous deformity correction trajectory.
Question 76
A patient exhibits a compensated Trendelenburg gait (abductor lurch) over the right lower extremity. How is the body's center of mass shifted during the stance phase to minimize the demands on the affected right hip abductors?
Explanation
In a compensated Trendelenburg gait, the patient leans the trunk laterally over the weak (right) hip. This shifts the center of gravity closer to the center of rotation of the hip joint, shortening the body weight lever arm and reducing the required abductor force.
Question 77
When evaluating a long bone uniapical deformity, the intersection of the proximal and distal anatomical axes defines the CORA. If a closing wedge osteotomy is executed with the hinge placed on the convex cortex exactly at the CORA, what is the effect on the mechanical axis and overall bone length?
Explanation
A closing wedge osteotomy hinged on the convex cortex at the CORA perfectly corrects the angular deformity, restoring collinearity to the mechanical axis. However, because a wedge of bone is removed without opening the opposite cortex, it inherently shortens the overall bone length.
Question 78
The patellofemoral joint reaction force is highly dependent on knee flexion angle and quadriceps muscle force. During which of the following activities does this force reach its highest magnitude?
Explanation
Patellofemoral joint reaction forces increase significantly with increasing degrees of knee flexion due to the required quadriceps force and the altered vector angle. It can reach up to 7 to 8 times body weight during a deep squat.
Question 79
In a standing long-leg anteroposterior radiograph of a normally aligned adult lower extremity, where does the mechanical axis line (drawn from the center of the femoral head to the center of the ankle plafond) pass in relation to the knee joint center?
Explanation
The mechanical axis of a normally aligned lower limb passes slightly medial to the center of the knee joint, typically by about 8 to 10 mm. This normal physiological alignment results in a slightly greater load distribution to the medial compartment of the knee.
Question 80
When performing a medial opening wedge high tibial osteotomy (HTO) for isolated medial compartment osteoarthritis, restoring the mechanical axis to intersect the tibial plateau at what percentage of its width (from medial to lateral) is recommended for optimal load redistribution?
Explanation
For medial compartment osteoarthritis, an HTO aims to shift the weight-bearing axis laterally to offload the diseased medial compartment. The target correction is a slight valgus overcorrection, intersecting the 62% to 66% coordinate of the tibial plateau width.
Question 81
A patient presents with a "high steppage" gait, characterized by excessive hip and knee flexion during the swing phase. This kinematic adaptation is most commonly a compensatory mechanism for which underlying muscular deficit?
Explanation
A high steppage gait compensates for a drop foot, which is caused by weakness or paralysis of the ankle dorsiflexors. To prevent the toes from dragging on the ground during the swing phase, the patient excessively flexes the hip and knee.
Question 82
A 12-year-old child undergoes a supramalleolar derotational osteotomy of the tibia for severe internal tibial torsion. If the fibula is left intact and not osteotomized, what is the most likely biomechanical complication at the ankle?
Explanation
Performing a distal tibial derotational osteotomy without addressing the fibula tethers the lateral side of the mortise. Externally rotating the distal tibia against an intact fibula pulls the lateral malleolus proximally, inducing an iatrogenic valgus tilt of the ankle joint.
Question 83
In severe adolescent Blount's disease, the complex multiplanar deformity of the proximal tibia classically includes varus angulation, internal tibial torsion, and which of the following sagittal plane abnormalities?
Explanation
Adolescent Blount's disease is characterized by a three-dimensional deformity of the proximal tibia: varus angulation, internal tibial torsion, and procurvatum (apex anterior angulation). This occurs because the posterior aspect of the proximal tibial physis continues to grow faster than the anterior aspect.
Question 84
According to Paley's Rule 2 of deformity correction, if an osteotomy is performed at a level distinct from the CORA, but the hinge axis is placed exactly on the CORA, what structural outcome will result upon angular correction?
Explanation
Paley's Rule 2 states that if the osteotomy is separate from the CORA but the hinge remains exactly on the CORA, the mechanical axis will be successfully realigned. However, the bone ends will translate relative to each other at the osteotomy site.
Question 85
The "windlass mechanism" of the foot provides necessary rigidity to the longitudinal arch for effective push-off. Which specific kinematic action primarily engages this mechanism during terminal stance?
Explanation
The windlass mechanism is activated by dorsiflexion of the first MTP joint during heel rise. This motion winds the plantar fascia around the metatarsal head, drawing the calcaneus toward the forefoot, elevating the medial arch, and creating a rigid lever for push-off.
Question 86
The anatomical axis of rotation of the talocrural (ankle) joint passes through the tips of the medial and lateral malleoli. Relative to the true mechanical and coronal planes of the tibia, how is this axis generally oriented?
Explanation
Because the lateral malleolus is positioned more posterior and distal than the medial malleolus, the true axis of the ankle joint is externally rotated (roughly 15 to 20 degrees) in the transverse plane and tilted laterally/downward in the coronal plane.
Question 87
During which of the following activities does the patellofemoral joint experience the highest joint reaction forces relative to body weight?
Explanation
The patellofemoral joint reaction force is highly dependent on the degree of knee flexion. Deep squatting generates the highest forces, reaching up to 7 to 8 times body weight. By comparison, descending stairs generates roughly 3 to 4 times body weight.
Question 88
According to Paley's principles of deformity correction, if an osteotomy is performed at a level separate from the center of rotation of angulation (CORA), and the hinge axis is also placed at the osteotomy site rather than the CORA, what is the resulting alignment?
Explanation
Under Paley's Rule 3, if both the osteotomy and the hinge are located at a level distinct from the CORA, the mechanical axes will not align collinearly. Instead, the proximal and distal axes become parallel, inadvertently creating a secondary translational deformity.
Question 89
A patient presents with a "foot slap" gait during the initial contact phase of walking. Weakness in which of the following muscles and what type of contraction is responsible for this abnormality?
Explanation
Foot slap occurs during the loading response phase due to an inability to control ankle plantarflexion. The tibialis anterior normally contracts eccentrically to lower the foot smoothly to the ground. Weakness in this eccentric contraction results in a rapid drop of the forefoot.
Question 90
A patient with severe right hip osteoarthritis uses a cane in the left hand during ambulation. Which of the following best describes the biomechanical rationale for the pain relief provided by this intervention?
Explanation
Using a cane in the contralateral hand creates a long moment arm to help counteract the torque produced by body weight. This significantly reduces the compensatory force required by the ipsilateral hip abductors. Consequently, the overall hip joint reaction force is substantially decreased.
Question 91
When evaluating a long standing radiograph for mechanical axis deviation (MAD), the mechanical axis line is drawn from the center of the femoral head to the center of the ankle mortise. In a patient with an uncompensated severe varus deformity of the lower extremity, where does the mechanical axis pass relative to the knee joint?
Explanation
The normal lower extremity mechanical axis passes slightly medial to the center of the knee joint (approximately 8 mm). In a severe varus deformity, the weight-bearing axis shifts further medially, placing increased stress on the medial compartment.
Question 92
During normal human gait, maximum knee flexion is required to allow for adequate foot clearance. At which phase of the gait cycle does maximum knee flexion occur, and what is the approximate degree of flexion?
Explanation
Maximum knee flexion during normal gait is essential for proper toe clearance to prevent tripping. This peaks during the initial swing phase, where the knee flexes to approximately 60 degrees.
Question 93
When utilizing the Ilizarov method for lengthening a tibia following a corrective osteotomy, what is the optimal rate and rhythm of distraction to promote high-quality regenerate bone formation?
Explanation
The standard rate for distraction osteogenesis is 1 mm per day. The optimal rhythm breaks this daily distance into smaller, frequent increments, typically 0.25 mm four times a day. This minimizes soft tissue trauma and yields the best quality regenerate bone.
Question 94
The "windlass mechanism" is critical for the biomechanics of the foot during the terminal stance phase of gait. Which of the following kinematic actions directly engages this mechanism?
Explanation
The windlass mechanism stabilizes the arch of the foot to create a rigid lever for push-off. Extension of the first metatarsophalangeal (MTP) joint tightly winds the plantar fascia around the metatarsal head, elevating the longitudinal arch.
Question 95
A patient presents with a procurvatum deformity of the tibial diaphysis. If an anterior closing wedge osteotomy is planned, where must the hinge axis be located relative to the apex of the deformity to avoid unintended shortening or lengthening of the mechanical axis?
Explanation
A procurvatum deformity has an anterior apex. To perform a closing wedge osteotomy without altering limb length, the wedge is removed from the convex (anterior) side, and the hinge must be placed exactly on the intact concave side, which is the posterior cortex.
Question 96
During normal human walking, the gastrocnemius-soleus complex provides the primary propulsive force. At which phase of the gait cycle do these ankle plantarflexors exhibit their peak electromyographic (EMG) activity?
Explanation
The gastrocnemius-soleus complex provides the power for push-off during normal gait. Peak EMG activity of the ankle plantarflexors occurs during terminal stance to stabilize the ankle and propel the body forward.
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Detailed Chapters & Topics
Dive deeper into specialized chapters regarding master-abos-board-review-part-7-2
01
Chapter 1
48 min
ABOS Board Review: Lower Extremity Deformity, TKA, & Gait Analysis | Part 5
02
Chapter 2
56 min
ABOS Orthopedic Board Review: Lower Extremity Deformity, Gait & Hip Biomechanics | Part 10
03
Chapter 3
55 min
Comprehensive Orthopedic Deformity, LLD & Hip Biomechanics Board Review | Part 13
04
Chapter 4
57 min
ABOS Orthopedic Board Review: Foot, Ankle & Hip Deformity Correction (Paley Principles) | Part 16
05
Chapter 5
52 min
Lower Extremity Deformity Correction & TKA Biomechanics for ABOS Board Review | Part 4
06
Chapter 6
16 min
Mastering Orthopedic Deformity Correction: Static Alignment to Dynamic Gait Biomechanics
07
Chapter 7
11 min
Cerebral Palsy: Comprehensive Diagnosis, Prognostic Evaluation, and Gait Analysis
08
Chapter 8
84 min
ABOS Part I Orthopaedic Surgery Board Exam Review: Lower Limb Deformity & Paley's Principles MCQs | Part 22028
09
Chapter 9
67 min
ABOS Part I & OITE Orthopedic Surgery Board Exam Review: Deformity Correction & Paley's Principles | Part 22016
10
Chapter 10
92 min
Comprehensive ABOS Part I Orthopedic Surgery Board Exam Review: Deformity Correction & Biomechanics | Part 22007
11
Chapter 11
62 min
ABOS Part I Orthopaedic Surgery Board Review: Deformity Correction, Paley's Principles & Fixation MCQs | Part 21961
12
Chapter 12
47 min
ABOS Part I Orthopedic Deformity Analysis & Surgical Planning Comprehensive Review | Part 21921
13
Chapter 13
72 min
ABOS Part I Orthopaedic Review: Paley Deformity Correction, Pelvic Support Osteotomy & Hip Principles | Part 21918
14
Chapter 14
56 min