Question 1

A patient presents with a gait abnormality characterized by a backward lurch of the trunk immediately after heel strike on the right side. Which muscle group is most likely weak?

Explanation

A backward trunk lean during early stance (gluteus maximus lurch) moves the center of mass posterior to the hip joint. This creates an artificial hip extension moment, compensating for weak hip extensors.

Question 2

A patient with a remote midshaft femur fracture malunion presents for TKA. Preoperative planning reveals a coronal plane extra-articular deformity. What is the generally accepted threshold for femoral extra-articular coronal deformity beyond which a concurrent osteotomy should be considered rather than an intra-articular compensatory resection?

Explanation

Intra-articular compensatory bone cuts in TKA are generally safe for extra-articular femoral deformities up to 20 degrees in the coronal plane. Deformities exceeding this typically require concurrent or staged extra-articular osteotomy to avoid excessive ligamentous imbalance.

Question 3

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) and the axis of correction (hinge) is placed at the osteotomy site rather than the CORA, what is the resulting effect?

Explanation

Paley's Rule 3 states that if the osteotomy and the hinge are both placed off the CORA, correction of angulation will create a secondary translation deformity. This is distinct from Rule 2, where the hinge remains at the CORA resulting in perfect axis alignment but local translation.

Question 4

During the second rocker (ankle rocker) phase of normal gait, which muscle group acts eccentrically to control the forward progression of the tibia over the foot?

Explanation

During the second rocker (mid-stance), the tibia advances over the plantigrade foot. The gastroc-soleus complex contracts eccentrically to control this forward tibial progression and prevent excessive ankle dorsiflexion.

Question 5

During a primary TKA for a severe varus deformity, the surgeon notes the knee remains tight medially in both flexion and extension. After completely releasing the deep medial collateral ligament (MCL), what is the next most appropriate structure to release?

Explanation

For a tight medial gap in both flexion and extension, the sequential release typically begins with osteophytes and the deep MCL. If tightness persists symmetrically, the posteromedial capsule is released next, followed by the semimembranosus and finally the superficial MCL.

Question 6

Elevation of the joint line during a revision TKA most commonly leads to which of the following postoperative complications?

Explanation

Elevating the joint line alters the kinematics of the knee by moving the femoral origin of the collateral ligaments distally relative to the joint line. This mismatch often results in laxity in mid-flexion (mid-flexion instability) and can cause an apparent patella baja.

Question 7

In evaluating a patient for lower extremity deformity, a standing full-length anteroposterior radiograph is obtained. The mechanical axis line passes medial to the center of the knee joint. The mechanical lateral distal femoral angle (mLDFA) is 95 degrees, and the medial proximal tibial angle (MPTA) is 87 degrees. What is the primary source of the varus deformity?

Explanation

A normal mLDFA is 85-90 degrees (average 88) and a normal MPTA is 85-90 degrees (average 87). An mLDFA of 95 degrees indicates excessive varus alignment originating in the distal femur, whereas the tibial MPTA is within normal limits.

Question 8

A 12-year-old child with spastic diplegic cerebral palsy presents with a profound crouch gait. Physical exam reveals severe flexion contractures of the knees. Over-lengthening of which muscle group during prior multi-level surgeries is the most common iatrogenic cause of this gait pattern?

Explanation

Iatrogenic crouch gait is frequently caused by over-lengthening of the Achilles tendon (gastrocnemius-soleus complex). This weakens plantarflexion, permitting excessive ankle dorsiflexion during stance, which biomechanically forces the knee and hip into secondary flexion.

Question 9

In a total knee arthroplasty, excessive internal rotation of the femoral component will most likely result in which of the following specific kinematic alterations?

Explanation

Internal rotation of the femoral component medializes the trochlear groove relative to the extensor mechanism. This increases the Q-angle vector and reliably leads to lateral patellar maltracking or subluxation.

Question 10

During limb lengthening of the tibia with a circular external fixator, radiographic follow-up at 3 weeks reveals premature consolidation of the regenerate bone. Which of the following adjustments to the protocol would have best prevented this complication?

Explanation

Premature consolidation occurs when the bone healing outpaces the distraction gap. Increasing the rate of distraction (e.g., from 1.0 mm/day to 1.5 mm/day) helps the mechanical distraction outpace the robust osteogenic response to prevent premature union.

Question 11

A patient exhibits a positive Trendelenburg sign on the right side. During the stance phase of gait on the right leg, what compensatory movement is typically observed to maintain stability?

Explanation

In an uncompensated Trendelenburg gait due to weak right hip abductors, the pelvis drops on the contralateral side. To compensate, the patient will lean their trunk toward the affected (right) side, shifting the center of gravity closer to the hip joint center to reduce the required abductor moment.

Question 12

Which of the following design modifications in a posterior-stabilized TKA helps substitute for the function of the resected posterior cruciate ligament (PCL)?

Explanation

In a posterior-stabilized TKA, the PCL is excised. A central post on the tibial polyethylene insert engages a cam on the femoral component during flexion, physically forcing femoral rollback and preventing anterior subluxation of the femur.

Question 13

A patient presents with a severe recurvatum (apex posterior) deformity of the proximal tibia following premature physeal closure. What effect does this specific deformity have on knee biomechanics and clinical presentation?

Explanation

A proximal tibial recurvatum deformity effectively increases the posterior slope of the tibial plateau. This alters the resting position of the knee, requiring greater quadriceps excursion to achieve full extension, thereby clinically manifesting as an apparent extensor lag.

Question 14

A patient with a dense common peroneal nerve palsy presents with a 'steppage' gait. During which phase of the gait cycle is the primary compensation (excessive hip and knee flexion) observed?

Explanation

A steppage gait is a compensation for foot drop (loss of ankle dorsiflexion) to prevent the toes from catching on the ground. Excessive hip and knee flexion are utilized for toe clearance predominantly during the swing phase.

Question 15

When planning a primary TKA for a patient with an extra-articular valgus tibial deformity, intra-articular resection is typically considered acceptable if the deformity is within what angular limit in the coronal plane?

Explanation

Intra-articular bone cuts can safely compensate for up to 10 to 15 degrees of extra-articular coronal plane deformity in the tibia. Deformities greater than 10-15 degrees risk compromising the tibial attachment of the collateral ligaments, usually necessitating an extra-articular osteotomy.

Question 16

During trial reduction in a cruciate-retaining TKA, the knee is perfectly balanced in full extension but significantly tight in flexion. Which of the following is the most appropriate surgical step to balance the knee?

Explanation

A knee that is balanced in extension but tight in flexion has an isolated tight flexion gap. Downsizing the femoral component (using an anterior referencing system) reduces the posterior condylar offset, increasing the flexion gap without altering the extension gap.

Question 17

A 65-year-old patient with severe hip abductor weakness demonstrates a compensated Trendelenburg gait. Which of the following biomechanical adaptations is characteristic during the stance phase of the affected limb?

Explanation

In a compensated Trendelenburg gait, the patient leans their trunk laterally over the affected hip during stance. This shifts the center of mass toward the joint, significantly reducing the moment arm and the force required by the weak hip abductors to maintain a level pelvis.

Question 18

During a cruciate-retaining total knee arthroplasty, trial reduction reveals a tight extension gap and a perfectly balanced flexion gap. What is the most appropriate next step to achieve a balanced knee?

Explanation

A tight extension gap with a balanced flexion gap requires increasing the extension space without altering the flexion space. This is best accomplished by releasing the posterior capsule or resecting additional distal femur.

Question 19

When planning a lower extremity deformity correction using an osteotomy, the osteotomy is placed proximal to the Center of Rotation of Angulation (CORA), but the angulation correction axis (ACA) passes directly through the CORA. Which of the following describes the resulting geometric outcome?

Explanation

According to the principles of deformity correction (Rule 2), if the ACA passes through the CORA but the osteotomy is made at a different level, the mechanical axis will be restored. However, this results in angulation combined with translation of the bone ends at the osteotomy site.

Question 20

During the normal human gait cycle, which muscle acts eccentrically to control the transition from initial contact to foot flat (the first rocker)?

Explanation

The first rocker (heel rocker) occurs from initial contact to foot flat. The tibialis anterior acts eccentrically during this phase to control plantarflexion and prevent the foot from slapping against the ground.

Question 21

In a primary total knee arthroplasty for a patient with a rigid 15-degree varus deformity, what is the generally accepted initial sequence of medial soft tissue release after removal of all osteophytes?

Explanation

For a fixed varus deformity, osteophytes are first removed. The initial sequential release typically involves the deep MCL, followed by the posteromedial capsule, and then the superficial MCL or pes anserinus if further correction is needed.

Question 22

A patient presents with knee pain and a suspected lower extremity malalignment. Full-length standing radiographs are obtained. Which of the following best defines the mechanical axis deviation (MAD)?

Explanation

Mechanical axis deviation (MAD) is mathematically defined as the perpendicular distance (in millimeters) from the center of the knee joint to the mechanical axis line extending from the center of the femoral head to the center of the ankle mortise.

Question 23

Which of the following is the most likely consequence of significantly elevating the joint line during a revision total knee arthroplasty?

Explanation

Elevating the joint line often requires a thicker polyethylene insert to balance extension, which can lead to mid-flexion instability because the collateral ligaments are relatively lax in mid-flexion. It also results in pseudo-patella baja, not alta.

Question 24

A 12-year-old with spastic diplegic cerebral palsy develops a worsening crouch gait following bilateral Achilles tendon lengthenings. Weakness or over-lengthening of which muscle group is the primary biomechanical cause of this resultant gait pattern?

Explanation

Over-lengthening the Achilles tendon weakens the plantarflexors, disrupting the normal plantarflexion-knee extension couple during the stance phase of gait. This failure to control forward tibial progression leads to excessive knee flexion, known as crouch gait.

Question 25

A patient requires a high tibial osteotomy for a varus deformity but also exhibits significant genu recurvatum. To simultaneously correct the varus malalignment and the recurvatum, how should the tibial slope be managed during the osteotomy?

Explanation

Genu recurvatum is associated with a decreased posterior tibial slope. To correct recurvatum during a high tibial osteotomy, the surgeon should increase the posterior tibial slope, which can be achieved through an anterior opening wedge technique.

Question 26

During a total knee arthroplasty, the femoral component is inadvertently placed in excessive internal rotation relative to the transepicondylar axis. Which of the following complications is most likely to occur?

Explanation

Internal rotation of the femoral component relative to the transepicondylar axis lateralizes the trochlear groove, increasing the Q angle and leading to lateral patellar subluxation. It also tightens the medial flexion gap and loosens the lateral flexion gap.

Question 27

During the loading response phase of a normal gait cycle, what is the position of the ground reaction force (GRF) vector relative to the hip, knee, and ankle joints?

Explanation

During loading response (early stance), the GRF vector passes anterior to the hip (causing a flexion moment), posterior to the knee (causing a flexion moment), and posterior to the ankle (causing a plantarflexion moment).

Question 28

A patient presents with a painful popping sensation at 30 degrees of flexion as they extend their knee from a fully flexed position, one year after a posterior-stabilized total knee arthroplasty. What is the most likely etiology?

Explanation

This classic presentation describes patellar clunk syndrome, which occurs in posterior-stabilized TKA. A fibrous nodule develops on the superior pole of the patella and catches in the intercondylar box of the femoral component as the knee extends from a flexed position.

Question 29

When comparing lengthening over a nail (LON) to traditional Ilizarov external fixation for femoral lengthening, which of the following is the primary established advantage of the LON technique?

Explanation

Lengthening over a nail (LON) allows the external fixator to be removed immediately after the distraction phase is complete, as the locked intramedullary nail supports the regenerate bone during the consolidation phase. This significantly reduces the total external fixation time.

Question 30

A patient with a painful TKA is being evaluated for a periprosthetic joint infection (PJI). Synovial fluid analysis reveals a highly elevated alpha-defensin level. What is the primary biological function of alpha-defensin in this context?

Explanation

Alpha-defensin is a biomarker released by neutrophils in response to pathogens. It serves as an antimicrobial peptide and is highly sensitive and specific for diagnosing periprosthetic joint infection.

Question 31

A patient with generalized lower extremity weakness is prescribed a solid ankle-foot orthosis (AFO) locked in 5 degrees of plantarflexion. How will this orthotic intervention primarily alter knee biomechanics during the stance phase of gait?

Explanation

An AFO set in plantarflexion limits forward progression of the tibia over the foot during the stance phase. This maintains the ground reaction force vector anterior to the knee, creating a knee extension moment which helps prevent knee buckling in patients with weak quadriceps.

Question 32

In a patient undergoing a primary total knee arthroplasty (TKA), the femoral component is inadvertently placed in excessive internal rotation. Which of the following postoperative clinical findings is most likely to occur?

Explanation

Internal rotation of the femoral component shifts the trochlear groove medially, which effectively increases the Q-angle. This alters patellofemoral kinematics, frequently resulting in lateral patellar tracking issues such as tilt, subluxation, or dislocation.

Question 33

A patient with an anterior cruciate ligament (ACL) deficient knee exhibits a specific gait adaptation to reduce anterior tibial translation during walking. Which of the following best describes the kinematics of this "quadriceps avoidance" gait?

Explanation

In a "quadriceps avoidance" gait, patients with ACL deficiency decrease normal knee flexion during the loading response (early stance). This minimizes quadriceps contraction, which would otherwise pull the tibia anteriorly and provoke instability.

Question 34

During a primary TKA, after making the standard bone cuts, the surgeon evaluates the gaps. The joint is tight in flexion and symmetric in extension. Which of the following is the most appropriate next surgical step?

Explanation

A gap that is tight in flexion but symmetric and well-balanced in extension requires an isolated increase in the flexion gap. This is best achieved by downsizing the femoral component or by increasing the posterior slope of the tibial cut.

Question 35

When performing a corrective osteotomy for a lower extremity angular deformity, placing the osteotomy and the mechanical hinge exactly at the Center of Rotation of Angulation (CORA) will result in which of the following?

Explanation

The CORA principle dictates that when the osteotomy and the mechanical hinge are both placed precisely at the CORA, pure angular correction is achieved without any resultant translation of the bone segments.

Question 36

According to Saunders' classic determinants of gait, which of the following kinematic mechanisms is primarily responsible for minimizing the vertical displacement of the center of gravity during the stance phase?

Explanation

Knee flexion during the stance phase (typically around 15 degrees) helps to lower the apex of the center of gravity's vertical excursion. This mechanism smoothes the pathway of the body's center of mass, thereby reducing overall energy expenditure.

Question 37

In the correction of a fixed valgus deformity during a total knee arthroplasty, the lateral compartment is found to be tight in full extension but balanced in 90 degrees of flexion. Which of the following structures is the primary tether and should be selectively released first?

Explanation

The iliotibial (IT) band primarily acts as a lateral tether in full extension but relaxes in flexion. If the lateral side is tight only in extension, isolated pie-crusting or release of the IT band is indicated.

Question 38

A 4-year-old girl is evaluated for a congenital femoral deficiency. Her projected limb length discrepancy at maturity is calculated to be 6 cm using the Paley multiplier method. Which parameter forms the foundational constant in the multiplier method algorithm?

Explanation

The multiplier method is based on the epidemiological observation that the ratio of a given bone's length at any specific chronological age to its final length at skeletal maturity is a constant. This allows for simple and accurate prediction of limb length discrepancy.

Question 39

A 14-year-old boy with spastic diplegic cerebral palsy presents with an increasingly severe crouch gait. He underwent isolated bilateral Achilles tendon lengthenings at age 6 for toe-walking. Which of the following best explains the pathophysiology of his current gait abnormality?

Explanation

Over-lengthening the Achilles tendon weakens the vital plantar flexor/knee extension couple. Without robust plantar flexors to restrain forward tibial progression during stance, the knee is forced into excessive flexion, causing iatrogenic crouch gait.

Question 40

During a complex revision TKA, the surgeon inadvertently elevates the joint line by 8 mm compared to the native knee. This technical error is most likely to result in which of the following biomechanical consequences?

Explanation

Elevating the joint line often necessitates using a thicker polyethylene insert to balance the extension gap. This alters the isometry of the collateral ligaments, leading to relative laxity and instability in mid-flexion, as well as patella baja.

Question 41

A patient undergoes a medial opening-wedge high tibial osteotomy (HTO) for a symptomatic varus knee deformity. Compared to a lateral closing-wedge HTO, the opening-wedge technique is inherently more likely to cause which of the following?

Explanation

Due to the triangular shape of the proximal tibia (narrower anteriorly than posteriorly), a medial opening-wedge HTO inherently increases the posterior tibial slope unless the anterior gap is intentionally kept smaller than the posterior gap.

Question 42

During the stance phase of gait, a patient demonstrates a pronounced lateral trunk lean towards the side of the supporting limb (gluteus medius lurch). This specific kinematic adaptation most effectively achieves which of the following biomechanical goals?

Explanation

A lateral trunk lean (Trendelenburg gait) shifts the body's center of gravity closer to the center of the stance-phase hip joint. This dramatically reduces the abductor muscle force required to maintain a level pelvis, thereby decreasing the overall hip joint reaction force.

Question 43

A patient presents with a painful "catch" and a palpable pop at the anterior aspect of the knee when extending from 40 degrees of flexion to full extension, 9 months following a posterior-stabilized TKA. What is the primary pathoanatomy driving this clinical presentation?

Explanation

Patellar clunk syndrome occurs primarily in posterior-stabilized TKA when a fibrous nodule forms on the posterior quadriceps tendon just superior to the patella. As the knee extends, this nodule catches in the intercondylar box of the femoral component and abruptly pops out.

Question 44

According to the classic Ilizarov principles of distraction osteogenesis for limb lengthening, which of the following represents the optimal daily rate and rhythm to promote robust regenerate formation while avoiding premature consolidation?

Explanation

The optimal rate of distraction is 1.0 mm per day, typically performed in four equal increments of 0.25 mm (rhythm). Slower rates risk premature consolidation, while faster rates or less frequent larger increments lead to poor regenerate formation and soft tissue complications.

Question 45

During normal human walking, the vertical ground reaction force (vGRF) curve typically exhibits a characteristic "double-hump" pattern. The two peaks of this curve correspond precisely to which events in the normal gait cycle?

Explanation

The "double-hump" vGRF pattern occurs due to weight acceptance during the loading response (first peak) and push-off during terminal stance (second peak). The trough between them represents mid-stance when the center of mass is at its highest point.

Question 46

When sizing the tibial component during a primary TKA, oversizing the component in the mediolateral dimension is most directly associated with which of the following postoperative complications?

Explanation

Mediolateral overhang of the tibial component frequently irritates the surrounding soft tissues, such as the medial collateral ligament (MCL) medially or the popliteus/IT band laterally. This impingement leads to persistent postoperative pain.

Question 47

You evaluate a 45-year-old with a post-traumatic varus deformity. On a full-length standing radiograph, the mechanical axis deviation (MAD) is medial. The mechanical lateral distal femoral angle (mLDFA) is 87 degrees, and the medial proximal tibial angle (MPTA) is 80 degrees.

Based on these measurements, what is the primary source of the deformity?

Explanation

Normal mLDFA is approximately 87 degrees (range 85-90), and normal MPTA is also 87 degrees (range 85-90). An MPTA of 80 degrees indicates an abnormal proximal tibia vara, making the tibia the source of the medial mechanical axis deviation.

Question 48

A 65-year-old female with severe unilateral hip osteoarthritis exhibits a pronounced Trendelenburg gait. Biomechanically, how does the observed lateral trunk shift during the stance phase of the affected limb alter the forces acting on the hip joint?

Explanation

A Trendelenburg (abductor lurch) gait compensates for weak hip abductors by laterally shifting the trunk over the stance limb. This moves the body's center of mass closer to the hip joint center, significantly reducing the external adductor moment and decreasing the demand on the deficient abductor muscles.

Question 49

During a primary total knee arthroplasty (TKA), trial reduction demonstrates a gap that is symmetric and well-balanced in full extension, but the knee is significantly tight in 90 degrees of flexion. Which of the following adjustments is the most appropriate next step to balance the knee?

Explanation

Downsizing the femoral component increases the flexion gap without altering the extension gap. Increasing the posterior tibial slope also increases the flexion gap but alters tibial bone stock, while resecting more proximal tibia affects both flexion and extension gaps symmetrically.

Question 50

A patient presents with a Trendelenburg gait due to severe hip abductor weakness. During the stance phase of the affected limb, what is the primary biomechanical purpose of the compensatory lateral trunk shift?

Explanation

By shifting the trunk laterally over the affected hip, the patient moves the center of mass closer to the center of the hip joint. This significantly decreases the lever arm of body weight, thereby reducing the torque that the weak abductors must counteract.

Question 51

According to the principles of deformity correction, if an osteotomy is performed at a level different from the center of rotation of angulation (CORA), but the hinge of the correction device is placed exactly at the CORA, what is the geometric result?

Explanation

Osteotomy Rule 2 states that if the hinge is at the CORA but the osteotomy is at a different level, the mechanical axis will realign collinearly. However, this inevitably results in translation of the bony fragments at the osteotomy site.

Question 52

During clinical gait analysis, a patient exhibits a 'foot slap' immediately after initial contact, transitioning into the loading response. Weakness of which of the following muscle groups is primarily responsible for this finding?

Explanation

During the loading response, the ankle dorsiflexors (primarily tibialis anterior) must contract eccentrically to smoothly lower the foot to the ground. Weakness in this eccentric contraction results in an abrupt 'foot slap'.

Question 53

A patient undergoes a revision total knee arthroplasty utilizing thick polyethylene inserts to address a severe flexion gap laxity, which ultimately elevates the joint line by 10 mm. Which of the following complications is most directly associated with this joint line elevation?

Explanation

Elevating the joint line during TKA lowers the patella relative to the new joint line, resulting in pseudo-patella baja. This alters patellofemoral tracking, limits knee flexion, and can increase anterior knee pain.

Question 54

When performing distraction osteogenesis using the Ilizarov method, what is the most widely accepted optimal rate and rhythm of distraction to promote high-quality regenerate bone while preventing premature consolidation?

Explanation

The classic Ilizarov principle recommends a distraction rate of 1.0 mm per day, optimally divided into four 0.25 mm increments. This rhythm maximizes osteogenesis and angiogenesis while minimizing soft tissue complications.

Question 55

At the exact moment of initial contact (heel strike) during normal walking gait, where does the ground reaction force (GRF) vector typically pass relative to the ankle and knee joints?

Explanation

At initial contact, the GRF vector passes posterior to the center of the ankle joint (creating a plantarflexion moment) and anterior to the knee joint center (creating an extension moment). These moments are counteracted by the anterior compartment musculature and hamstrings, respectively.

Question 56

During a primary TKA for a severe varus deformity, step-wise medial soft tissue release is necessary. After releasing the deep medial collateral ligament (MCL) and removing osteophytes, the knee remains tight in extension. Which structure should typically be released next to address this extension-predominant tightness?

Explanation

The posteromedial capsule is a primary restraint to knee extension on the medial side. Releasing it selectively corrects a tight medial extension gap more than the flexion gap, preserving the superficial MCL for coronal stability if possible.

Question 57

Anterior opening-wedge high tibial osteotomy (HTO) is planned to correct a severe genu recurvatum deformity. If the hinge is placed too far anteriorly, which of the following is the most likely consequence regarding the patellofemoral joint?

Explanation

An anterior opening-wedge osteotomy increases the posterior slope and lengthens the anterior cortex. This effectively distalizes the tibial tubercle relative to the joint line, increasing the risk of iatrogenic patella baja.

Question 58

During the loading response phase of a normal gait cycle, what is the primary role of the quadriceps muscle group?

Explanation

During the loading response (weight acceptance), the knee flexes approximately 15 degrees. The quadriceps contract eccentrically to control this flexion, which serves as a crucial shock-absorbing mechanism.

Question 59

In a patient who recently underwent total knee arthroplasty, combined internal rotation malalignment of both the femoral and tibial components will most likely result in which of the following?

Explanation

Internal rotation of the femoral and tibial components increases the Q-angle dynamically. This creates a severe laterally directed force vector on the extensor mechanism, leading to lateral patellar subluxation or dislocation.

Question 60

In a normally aligned lower extremity, the mechanical axis (a line drawn from the center of the femoral head to the center of the ankle mortise) passes through the knee joint at which location?

Explanation

The normal mechanical axis of the lower extremity does not pass exactly through the center of the knee. It typically passes approximately 8 mm medial to the center of the knee joint, placing slightly more load on the medial compartment.

Question 61

A 12-year-old with spastic diplegic cerebral palsy presents with a severe crouch gait. Kinematically, this gait pattern is characterized by which combination of joint positions during the stance phase?

Explanation

Crouch gait is defined by excessive hip flexion, knee flexion, and ankle dorsiflexion during stance. It often develops as a result of hamstring tightness combined with over-lengthened Achilles tendons from previous surgeries.

Question 62

The primary objective of using 'kinematic alignment' principles rather than traditional 'mechanical alignment' in total knee arthroplasty is to:

Explanation

Kinematic alignment aims to restore the patient's pre-arthritic native joint lines and normal knee kinematics by co-aligning the components with the three kinematic axes of the knee, minimizing the need for soft tissue releases.

Question 63

When utilizing a computer-assisted hexapod circular external fixator (e.g., Taylor Spatial Frame) for complex lower extremity deformity correction, which of the following parameters is an absolute prerequisite to generate an accurate software prescription?

Explanation

The hexapod software calculates strut adjustments based on the exact spatial relationship between the reference ring and the bone. Inaccurate mounting parameters (AP, lateral, and axial offsets) will lead to an incorrect prescription and malalignment.

Question 64

According to the determinants of normal gait, which of the following kinematic mechanisms is most responsible for minimizing the superior vertical excursion of the body's center of mass during mid-stance?

Explanation

Pelvic drop (tilt on the swing side) and knee flexion during stance both serve to lower the peak vertical height of the center of mass during mid-stance. This conserves energy by reducing vertical displacement.

Question 65

During a TKA for a severe, fixed valgus knee deformity, a lateral soft tissue release is required. If the knee is found to be tight exclusively in flexion but balanced in extension, which lateral structure is the most appropriate initial target for release?

Explanation

The popliteus tendon is a primary restraint to flexion on the lateral side of the knee. Releasing the popliteus preferentially opens the lateral flexion gap, whereas releasing the ITB preferentially affects the lateral extension gap.

Question 66

When executing an acute, large-magnitude closing-wedge proximal tibial osteotomy for adolescent Blount's disease, a concurrent fibular osteotomy or proximal tibiofibular joint release is mandated primarily to:

Explanation

The intact fibula acts as a strong lateral strut. Attempting a significant angular correction of the tibia without addressing the fibula leads to mechanical tethering, preventing complete correction and placing excessive stress on the proximal tibiofibular joint.

Question 67

A patient recovering from a traumatic brain injury walks with a 'stiff-knee' gait, characterized by significantly diminished knee flexion during the swing phase. Overactivity of which muscle is the most common etiology of this specific gait abnormality?

Explanation

Stiff-knee gait in upper motor neuron lesions is classically caused by inappropriate, prolonged firing of the rectus femoris during the swing phase. This prevents the normal passive knee flexion required for foot clearance.

Question 68

In a mechanically well-aligned, perfectly balanced TKA utilizing conventional ultra-high-molecular-weight polyethylene (UHMWPE), what is the primary tribological wear mechanism that generates the submicron particles responsible for osteolysis?

Explanation

Adhesive wear occurs during normal articulation as microscopic asperities on the articular surfaces bond and break apart. This is the primary mechanism generating millions of submicron UHMWPE particles, which are the main culprits in macrophage-mediated osteolysis.

Question 69

A patient with a history of poliomyelitis walks with a forward lean of the trunk during the early stance phase of gait. Which underlying muscular deficit is this compensatory mechanism attempting to overcome?

Explanation

A forward trunk lean during early stance shifts the body's center of gravity anterior to the knee joint center. This creates a mechanical extension moment at the knee, compensating for weak quadriceps and preventing knee buckling.

Question 70

When planning a total knee arthroplasty in a patient with an extra-articular diaphyseal malunion, what is the generally accepted threshold for a coronal plane tibial deformity beyond which a concurrent tibial osteotomy is recommended over an intra-articular compensatory resection?

Explanation

For tibial extra-articular deformities, a coronal deformity >10 to 15 degrees generally dictates a concurrent osteotomy. Intra-articular correction of larger deformities risks asymmetric bone resection and compromise of the collateral ligament insertions.

Question 71

Based on Paley's rules of deformity correction, if an osteotomy is created at a site distant from the center of rotation of angulation (CORA), but the hinge (axis of correction) is placed exactly at the CORA, what is the geometric result?

Explanation

Paley's Rule 2 states that if the osteotomy is distinct from the CORA but the hinge is precisely at the CORA, the mechanical axes will realign to be collinear. However, this geometry necessitates translation at the osteotomy site itself.

Question 72

During the stance phase of normal human gait, the 'second rocker' mechanism involves forward translation of the tibia over a plantigrade foot. Which muscle group predominantly controls the rate of this progression via eccentric contraction?

Explanation

The ankle rocker (second rocker) occurs during mid-stance as the tibia advances over the fixed foot. The triceps surae (gastrocnemius and soleus) contract eccentrically to control this forward tibial progression and prevent collapse.

Question 73

A total knee arthroplasty is planned using conventional instrumentation on a patient with excessive anterior bowing of the femoral diaphysis. If a standard long straight intramedullary alignment rod is used to direct the distal femoral cut, what is the most likely error in component positioning?

Explanation

A straight intramedullary rod placed into an excessively anterior-bowed femur will dictate a relatively extended distal femoral cut compared to the true mechanical axis. This results in the femoral component being placed in excessive flexion.

Question 74

A 45-year-old patient has a 4 cm true limb length discrepancy secondary to a remote femur fracture. What is the most common kinematic compensation observed in the shorter limb during the stance phase of gait?

Explanation

To functionally lengthen a short limb, patients commonly walk with the ankle in an equinus position (plantarflexion) during stance. Conversely, they compensate on the longer limb by increasing hip and knee flexion during stance and swing phases.

Question 75

During a revision total knee arthroplasty, excessive distal femoral bone loss is managed with a standard femoral component and a thick polyethylene insert, resulting in a 12 mm elevation of the joint line. Which of the following is the most likely postoperative clinical consequence?

Explanation

Elevating the joint line alters the isometry of the collateral ligaments, frequently leading to mid-flexion instability. It also moves the joint line closer to the tibial tubercle, causing pseudo-patella baja which limits knee flexion.

Question 76

On a standardized long-leg standing anteroposterior radiograph, the mechanical axis of the lower extremity is drawn from the center of the femoral head to the center of the ankle mortise. In a normal knee, where does this line typically pass?

Explanation

The normal mechanical axis of the lower extremity demonstrates a slight physiologic varus relative to the joint center. It typically passes approximately 8 mm (+/- 7 mm) medial to the absolute geometric center of the knee.

Question 77

Normal gait requires adequate knee flexion during the swing phase to ensure foot clearance. What is the maximum degree of knee flexion typically achieved during the normal human gait cycle?

Explanation

During the swing phase of normal gait, the knee flexes to a maximum of approximately 60 to 65 degrees. This peak flexion occurs during initial swing and is crucial for toe clearance.

Question 78

A surgeon plans an intra-articular compensatory bone cut to correct a 12-degree extra-articular diaphyseal tibial varus deformity during a TKA. What is the primary risk associated with making an aggressively asymmetric intra-articular tibial resection to achieve a neutral mechanical axis?

Explanation

Compensating for a large extra-articular varus deformity intra-articularly requires removing a much larger wedge of lateral bone relative to the medial side. If the deformity is severe, making the required minimal medial cut risks completely detaching the superficial MCL insertion.

Question 79

During normal human walking, what is the position of the ground reaction force (GRF) vector relative to the hip, knee, and ankle joints immediately at initial contact (heel strike)?

Explanation

At initial contact, the foot is anterior to the center of mass. The ground reaction force vector passes anterior to the hip (flexion moment), anterior to the knee (extension moment), and posterior to the ankle (plantarflexion moment).

Question 80

A 65-year-old patient presents for TKA with a 15-degree extra-articular varus deformity in the proximal tibial diaphysis due to a previous fracture. If a compensatory intra-articular resection is performed instead of an extra-articular osteotomy, which of the following ligamentous complexes will most likely require extensive release to balance the knee?

Explanation

Compensating for a large diaphyseal varus deformity with an intra-articular cut perpendicular to the mechanical axis requires significant resection of the lateral tibial plateau. This creates relative laxity on the lateral side, obligating extensive release of the medial structures (SMCL) to balance the gaps.

Question 81

A 12-year-old patient with spastic diplegic cerebral palsy presents with a progressive crouch gait following bilateral Achilles tendon lengthening procedures performed two years ago. Which of the following biomechanical abnormalities is the primary cause of this patient's current gait pattern?

Explanation

Over-lengthening the Achilles tendon weakens the gastroc-soleus complex, disrupting the plantarflexion-knee extension couple during stance phase. This leads to unchecked anterior tibial advancement, increased knee flexion, and an iatrogenic crouch gait.

Question 82

According to Paley's rules of deformity correction, if an osteotomy is performed at a level distinct from the center of rotation of angulation (CORA), but the axis of correction (hinge) is placed exactly at the CORA, what is the resultant geometric effect on the bone?

Explanation

Paley's Rule 2 states that when the osteotomy is separate from the CORA but the hinge remains at the CORA, the mechanical axis will be completely realigned. However, this geometric arrangement obligates a collinear translation of the bone ends at the osteotomy site.

Question 83

During a revision total knee arthroplasty, the joint line is inadvertently elevated by 10 mm. Which of the following kinematic changes or clinical complications is most likely to occur as a direct result of this alteration?

Explanation

Elevating the joint line alters patellofemoral mechanics by creating a pseudo-patella baja, leading to increased contact stresses and patellar instability. It also alters collateral ligament isometry, frequently resulting in mid-flexion coronal instability.

Question 84

During the initial swing phase of normal gait, which muscle group serves as the primary accelerator to advance the lower extremity forward?

Explanation

The hip flexors (primarily the iliopsoas) contract concentrically during the initial swing phase to accelerate the leg forward. This action is critical for advancing the limb and achieving foot clearance.

Question 85

A patient presents with a severe valgus deformity of the lower extremity. Radiographic analysis reveals a mechanical lateral distal femoral angle (mLDFA) of 80 degrees and a medial proximal tibial angle (MPTA) of 95 degrees. What is the most appropriate surgical strategy for correcting the mechanical axis deviation?

Explanation

Normal mLDFA and MPTA are both approximately 87 degrees. An mLDFA of 80 degrees indicates a valgus femur, and an MPTA of 95 degrees indicates a valgus tibia. This multi-apical deformity requires varus osteotomies at both levels to restore the mechanical axis while keeping the joint line horizontal.

Question 86

A 62-year-old male presents for a total knee arthroplasty five years after undergoing a medial opening-wedge high tibial osteotomy (HTO). Which of the following technical challenges is most frequently encountered during TKA in this specific patient population compared to primary TKA in a patient without prior surgery?

Explanation

A medial opening-wedge HTO inherently increases the posterior tibial slope and often results in patella baja. During a subsequent TKA, the surgeon must correct this exaggerated slope, typically requiring more extensive bone resection from the posterior aspect of the proximal tibia.

Question 87

During the stance phase of normal gait, what is the coupled motion of the subtalar joint and the tibia from initial contact to midstance?

Explanation

From initial contact to midstance, the subtalar joint acts as a shock absorber by everting (pronating), unlocking the transverse tarsal joint for foot flexibility. This motion is biomechanically coupled with obligatory internal rotation of the tibia.

Question 88

When applying a circular external fixator (Ilizarov) for the treatment of a tibial nonunion, which of the following modifications most significantly increases the axial stiffness of the frame?

Explanation

The axial stiffness of a circular external fixator is highly dependent on frame geometry. Using the smallest possible ring diameter (decreasing ring-to-bone distance) significantly increases frame stability and axial stiffness.

Question 89

A 45-year-old stroke patient exhibits a stiff-knee gait characterized by severely reduced knee flexion during the swing phase. Overactivity of which of the following muscles is the most common primary contributor to this specific gait abnormality?

Explanation

Stiff-knee gait in stroke and cerebral palsy patients is predominantly caused by spasticity or prolonged firing of the rectus femoris during the pre-swing and initial swing phases. This overactivity restricts the passive knee flexion required for foot clearance.

Question 90

During a total knee arthroplasty in a patient with a severe, fixed valgus deformity, the extension gap remains tight laterally, while the flexion gap is well-balanced. Which of the following structures is the primary tether causing the lateral tightness specifically in extension, and should be the initial target for release?

Explanation

In a valgus knee, the iliotibial band is a major lateral deforming force that becomes notably tight in extension but relaxes in flexion. The popliteus tendon and lateral collateral ligament are primarily responsible for lateral tightness in flexion.

ABOS Part I Orthopaedic Deformity Correction, Limb Reconstruction & Gait Analysis Review | Part 21914

ABOS Board Review: Lower Extremity Deformity, TKA, & Gait Analysis | Part 5

Key Takeaway

ABOS Board Review: Lower Extremity Deformity, TKA, & Gait Analysis | Part 5

Chapter Index