ABOS Orthopedic Board Review: Foot, Ankle & Hip Deformity Correction (Paley Principles) | Part 16

Correct Answer: C

Rationale: The Center of Rotation of Angulation (CORA) is the point where the proximal and distal mechanical axes of a deformed bone intersect. Placing the osteotomy hinge at the CORA allows for pure angular correction without causing unwanted translation of the distal fragment. Placing the hinge proximal or distal to the CORA would induce translation along with the angular correction.

Correct Answer: B

Rationale: A medializing calcaneal osteotomy shifts the weight-bearing tuberosity of the calcaneus medially. This realigns the ground reaction force vector to be medial to the subtalar joint axis, thereby converting the deforming valgus moment of the Achilles tendon into a stabilizing varus moment. Lengthening the lateral column is achieved with an Evans osteotomy, not a medializing calcaneal osteotomy.

Correct Answer: B

Rationale: For a varus deformity, the apex of the deformity (CORA) is on the lateral side. To correct this with a medial opening wedge osteotomy, the hinge must be placed at the CORA on the lateral cortex to allow for angular correction without translation. Placing the hinge on the medial cortex would be for a closing wedge osteotomy to correct a valgus deformity.

Correct Answer: D

Rationale: The medializing calcaneal osteotomy is typically performed through a lateral incision. However, the medial displacement of the posterior tuberosity can lead to tension or direct compression of the tibial nerve and its branches (medial and lateral plantar nerves) as they pass through the tarsal tunnel on the medial side of the ankle. The sural nerve is at risk with the lateral incision itself, but plantar numbness points to the tibial nerve or its branches.

Correct Answer: C

Rationale: The CORA is a fundamental concept in deformity correction. It is defined as the point of intersection of the proximal mechanical axis and the distal mechanical axis of a deformed bone. All deformity correction planning revolves around identifying the CORA to plan the osteotomy level and type. The other options are related but not the direct definition of this intersection point.

Correct Answer: C

Rationale: In a cavovarus foot, the calcaneus is in varus, and the Achilles tendon vector pulls medially, exacerbating the deformity. A lateralizing calcaneal osteotomy shifts the calcaneal tuberosity laterally, moving the Achilles insertion lateral to the subtalar joint axis. This converts the Achilles from a varus deforming force to a valgus-correcting force. A medializing osteotomy, as depicted for a flatfoot, would worsen the varus deformity.

Correct Answer: A

Rationale: When an angular correction is performed at a level other than the CORA, a translation is induced. For a valgus deformity corrected with a medial opening wedge, if the osteotomy is proximal to the CORA, the distal fragment will translate medially. This is known as the "Rule of Thumb" in deformity planning. Correction at the CORA avoids this translation.

Correct Answer: C

Rationale: The standard approach for a calcaneal osteotomy is through a lateral incision, posterior and inferior to the fibula. The sural nerve runs in close proximity to this incision and is the structure most at risk of iatrogenic injury during the surgical approach. The tibial nerve and posterior tibial tendon are on the medial side and are at risk from over-translation or hardware, but not from the initial approach.

Correct Answer: A

Rationale: For a valgus deformity, the apex of the deformity (CORA) is on the medial side. A closing wedge osteotomy removes a wedge of bone from the lateral side. To achieve pure angular correction, the hinge, or apex of the wedge, must be placed at the CORA. Therefore, the apex should be on the medial cortex at the level of the CORA.

Correct Answer: D

Rationale: In a valgus hindfoot, the Achilles tendon insertion is lateral to the subtalar joint axis, creating a valgus-deforming moment arm. By medializing the calcaneal tuberosity, the insertion of the Achilles tendon (part of the gastrocnemius-soleus complex) is shifted medially. This reduces or even reverses the valgus moment arm, making the Achilles a neutral or varus-producing force, which aids in correcting the deformity.

Correct Answer: C

Rationale: When the CORA is intra-articular and significant arthritis is present, a joint-preserving osteotomy (like a supramalleolar osteotomy) is contraindicated. The goal of an osteotomy is to realign a well-preserved joint. With an intra-articular deformity and advanced arthritis, the treatment must address the joint itself, making ankle arthrodesis or total ankle arthroplasty the most appropriate options.

Correct Answer: B

Rationale: Large acute corrections, especially lengthening osteotomies of the tibia, can significantly increase the pressure within the fascial compartments of the lower leg. An anterior opening wedge osteotomy for procurvatum effectively lengthens the anterior aspect of the tibia, putting the anterior compartment at high risk. Prophylactic fasciotomy is often performed to prevent acute compartment syndrome.

Correct Answer: C

Rationale: The Evans osteotomy is an opening wedge osteotomy of the anterior process of the calcaneus. It lengthens the lateral column of the foot, which corrects forefoot abduction and helps to rotate the transverse tarsal joint back into a locked, stable position. It is a powerful tool for correcting the transverse plane deformity seen in flexible flatfoot. The Cotton osteotomy addresses forefoot supination, not abduction.

Correct Answer: B

Rationale: A hindfoot fixed in excessive valgus after a triple arthrodesis can lead to subtalar or sinus tarsi impingement, where the lateral process of the talus and the calcaneus impinge on the floor of the sinus tarsi and the distal fibula. This causes characteristic lateral-sided pain. Sural nerve neuritis is a risk of the incision, but impingement is a direct result of the valgus malunion.

Correct Answer: C

Rationale: The fundamental principle of CORA-based deformity correction is that performing the osteotomy and correction at the level of the CORA results in pure angular correction. If the osteotomy is performed proximal or distal to the CORA, a translational deformity is induced in addition to the angular correction, which can lead to joint incongruity and poor outcomes.

Correct Answer: D

Rationale: While sural nerve injury is a risk of the approach, a common complication specific to the lateralizing osteotomy is wound tension and subsequent dehiscence. The lateral shift of the large posterior calcaneal fragment puts significant tension on the lateral skin flap, which can compromise wound healing. This is a key consideration during closure.

Correct Answer: B

Rationale: The primary goal of Charcot foot reconstruction is not to restore perfect anatomy, but to create a stable, plantigrade foot that is free of bony prominences that could cause ulceration. This allows the patient to wear accommodative footwear or bracing and remain ambulatory. While ulcer debridement is necessary, it does not address the underlying structural deformity.

Correct Answer: D

Rationale: Forefoot supination is a compensatory deformity in flexible flatfoot that can become fixed. After the hindfoot is corrected (e.g., with a calcaneal osteotomy), this supination can manifest as elevation of the first ray. The Cotton osteotomy, a dorsal opening wedge osteotomy of the medial cuneiform, specifically plantarflexes the first ray to restore the medial column tripod and correct the residual forefoot deformity.

Correct Answer: C

Rationale: In a supramalleolar osteotomy, the distal tibial fragment is realigned. The fibula, however, remains attached to the talus via the lateral ligaments. To allow the talus to move with the distal tibia and maintain a congruent ankle mortise, the fibula must be osteotomized. It is typically fixed in a new position that matches the tibial correction.

Correct Answer: D

Rationale: In a varus hindfoot, the Achilles tendon pulls medially, worsening the deformity. A lateralizing calcaneal osteotomy shifts the insertion of the Achilles tendon lateral to the axis of the subtalar joint. This changes the pull of the gastrocnemius-soleus complex from a deforming varus force to a corrective valgus force, which helps to stabilize the hindfoot.

Correct Answer: B

Rationale: According to the provided text, the Mechanical Axis Deviation (MAD) is the first and most critical global measurement to assess the entire lower extremity. It identifies any proximal deformity that alters ground reaction forces on the foot and ankle, which must be understood before addressing the local ankle deformity. The LDTA is a local measurement that is assessed after the global alignment is understood.

Correct Answer: B

Rationale: The line described is the mechanical axis. In a normally aligned limb, it passes just medial to the center of the knee. Significant deviation, as described, indicates a proximal deformity (e.g., in the femur or proximal tibia) that is contributing to the overall malalignment and must be considered in the treatment plan. This is the definition and purpose of assessing the Mechanical Axis Deviation (MAD).

Correct Answer: C

Rationale: The text states that the normal LDTA is approximately 89°. A value greater than 92° indicates a valgus deformity. Therefore, an LDTA of 95° represents a valgus deformity of the distal tibia. A varus deformity would be indicated by an LDTA less than 86°.

Correct Answer: D

Rationale: The normal Anterior Distal Tibial Angle (ADTA) is 80°. According to the text, a value less than 80° indicates procurvatum, which is an anterior angulation or apex anterior deformity. Recurvatum would be indicated by an ADTA greater than 80°.

Correct Answer: C

Rationale: The text specifies that the normal Joint Line Congruency Angle (JLCA) is 0-2°. An abnormal JLCA, such as 8°, suggests that the source of the deformity is intra-articular, stemming from issues like cartilage loss or significant ligamentous instability, rather than a purely bony deformity in the tibial metaphysis (which would be identified by an abnormal LDTA with a normal JLCA).

Correct Answer: C

Rationale: The provided text explicitly defines the CORA as "the precise mathematical point where the proximal anatomical axis and the distal anatomical axis of a deformed bone intersect." This is the foundational concept for planning an osteotomy to achieve accurate correction.

Correct Answer: C

Rationale: Paley's Rule 1, which describes the ideal scenario for pure angular correction, states that when the osteotomy passes through the CORA and the hinge is placed exactly at the CORA, the result is pure angular correction with no secondary translation. This is the most desirable technique for most supramalleolar osteotomies.

Correct Answer: C

Rationale: This scenario describes Paley's Rule 2. When the osteotomy is made outside the CORA (in this case, to protect the physis, a "no-cut zone"), the hinge must still be placed at the CORA to achieve angular correction with predictable translation that realigns the axis. Placing the hinge at the osteotomy site would be an error (Rule 3).

Correct Answer: D

Rationale: This is a classic example of Paley's Rule 3. When the osteotomy is made outside the CORA and the hinge is also placed outside the CORA (in this case, at the osteotomy site), the result is angular correction with an unintended secondary translational deformity. This is generally considered a surgical error.

Correct Answer: B

Rationale: The text provides a specific example for locating the CORA in a rigid equinus deformity. It states that "the CORA is often located at the intersection of the tibial axis and a line parallel to the plantar surface of the foot." This identifies the apex of the sagittal plane deformity.

Correct Answer: C

Rationale: The normal LDTA is approximately 89°. The text specifies that a value less than 86° indicates a varus deformity. An LDTA of 78° is well below this threshold, confirming a significant varus deformity in the coronal plane that would necessitate a supramalleolar osteotomy.

Correct Answer: C

Rationale: The normal ADTA is 80°. The text states that a value greater than 80° indicates recurvatum (posterior angulation, or apex posterior). Among the options, 90° is the only value greater than 80° and is therefore consistent with a recurvatum deformity. An ADTA of 75° would indicate procurvatum.

Correct Answer: B

Rationale: The text explicitly states, "The foundational philosophy of the Paley method is the exact quantification of deformity through standardized radiographic measurements. This mathematical approach replaces intraoperative guesswork with a predictable, reproducible, and highly accurate surgical plan."

Correct Answer: D

Rationale: The text warns, "Attempting to correct a foot deformity without addressing a massive MAD is a recipe for recurrent deformity and failure." The uncorrected proximal varus will continue to alter ground reaction forces, forcing the foot to compensate and causing the surgically corrected deformity to recur over time.

Correct Answer: D

Rationale: The text identifies the Lateral Distal Tibial Angle (LDTA) as "the key coronal plane measurement." It further states that this angle "dictates the necessity and magnitude of a supramalleolar osteotomy (SMO)." The ADTA is for the sagittal plane, and the JLCA assesses the joint surface itself.

Correct Answer: D

Rationale: The LDTA (89°) and ADTA (80°) are normal, indicating no significant bony malalignment in the distal tibial metaphysis. However, the JLCA is significantly abnormal (normal is 0-2°). This combination pinpoints the problem to the tibiotalar joint itself, suggesting an intra-articular issue like cartilage loss or instability.

Correct Answer: C

Rationale: The text states, "Correctly identifying the CORA dictates where the bone should be cut (the osteotomy) and where the hinge of an external fixator must be placed to achieve pure angular correction without unwanted translation."

Correct Answer: C

Rationale: Paley's Rule 1 is defined by two conditions for achieving pure angular correction: the osteotomy must pass through the CORA, and the hinge must be placed exactly at the CORA. This combination prevents any secondary translational deformity.

Correct Answer: C

Rationale: The text explains that Rule 2 is essential when the CORA is located in a "no-cut zone," such as the middle of the ankle joint or an open physis. The osteotomy is moved to a safe location (e.g., the metaphysis), but the hinge remains at the joint CORA to correctly realign the axis, resulting in angular correction with a predictable translation.

Correct Answer: B

Rationale: The text states, "The ultimate goal of any comprehensive lower limb reconstruction is to restore a neutral mechanical axis." This ensures that ground reaction forces are transmitted appropriately through the limb, preventing the recurrence of deformity and promoting long-term joint health.

Correct Answer: A

Rationale: The text defines the ideal scenario of "pure angular correction" as the outcome of Paley's Rule 1. This is achieved when both the osteotomy and the external fixator hinge are placed at the CORA, preventing any unwanted translation during the correction.

Correct Answer: C

Rationale: According to the provided text, locating the CORA is the critical first step that dictates the entire surgical strategy for deformity correction. The other options are subsequent steps in the planning or execution process.

Correct Answer: B

Rationale: The Lateral Distal Tibial Angle (LDTA) is defined as the lateral angle formed by the intersection of the tibial mechanical axis and the ankle joint line on a frontal (coronal) plane radiograph. The ADTA is measured on the sagittal view.

Correct Answer: C

Rationale: The normal value range for the LDTA is 86° to 92°, with an average of 89°. Therefore, a measurement of 89° is considered normal alignment in the coronal plane. Varus would be indicated by a value less than 86°, and valgus by a value greater than 92°.

Correct Answer: D

Rationale: A valgus deformity of the distal tibia is characterized by an LDTA greater than the normal range of 86°-92°. An LDTA of 98° represents a significant valgus malalignment. An LDTA of 75° would indicate a varus deformity.

Correct Answer: D

Rationale: The text and accompanying diagram explicitly define the Center of Rotation of Angulation (CORA) as the geometric apex of the deformity, which is the precise point where the proximal and distal mechanical axes of a deformed bone intersect.

Correct Answer: A

Rationale: This scenario describes Paley's Osteotomy Rule 1. When the osteotomy and the corrective hinge are both placed at the CORA, pure angular correction occurs without any unwanted translation. This is considered the ideal correction.

Correct Answer: D

Rationale: This is an application of Paley's Osteotomy Rule 2. When the osteotomy is performed away from the CORA but the hinge is placed at the CORA, the correction requires both angulation and translation to realign the mechanical axis perfectly.

Correct Answer: C

Rationale: The Anterior Distal Tibial Angle (ADTA) is the angle measured on a sagittal view, defined by the intersection of the tibial mechanical axis and the ankle joint line. The LDTA is measured on the coronal view.

Correct Answer: C

Rationale: The normal range for the ADTA is 78° to 82°. A value less than this range, such as 72°, indicates a procurvatum or equinus-type deformity, where the distal tibia is angled anteriorly, limiting dorsiflexion. Recurvatum would be an ADTA greater than 82°.

Correct Answer: D

Rationale: This scenario describes Paley's Osteotomy Rule 3. When both the osteotomy and the hinge are placed away from the CORA, an undesirable secondary translation deformity will be induced, complicating the correction and leading to a non-anatomic result.

Correct Answer: B

Rationale: The normal LDTA is 86°-92°. A value of 78°, which is less than the normal range, signifies a varus deformity originating from the distal tibia. While subtalar varus may also be present, the abnormal LDTA confirms a tibial etiology.

Correct Answer: C

Rationale: The text defines the mechanical axis of the lower extremity as a line drawn from the center of the femoral head to the center of the ankle mortise. This line should normally pass through the center of the knee.

Correct Answer: C

Rationale: The normal range for the ADTA is 78° to 82°. A measurement of 81° falls within this normal range, indicating that there is no significant sagittal plane deformity at the level of the distal tibia.

Correct Answer: B

Rationale: The clinical image shows a significant valgus deformity of the ankle. This coronal plane deformity would be reflected on radiographs as an abnormally high LDTA, which is normally 86°-92°. A value of 100° is consistent with valgus. An LDTA of 80° would indicate varus.

Correct Answer: C

Rationale: The text states that malorientation of the distal tibia leads to abnormal joint contact pressures, accelerated cartilage degeneration, and a cascade of compensatory deformities in the subtalar joint and midfoot. The most direct and primary consequence is the altered mechanics within the ankle joint itself.

Correct Answer: B

Rationale: The normal ADTA is 78°-82°. A value of 90° is greater than normal, indicating a recurvatum deformity (dorsiflexion deformity) of the distal tibia. This can limit plantarflexion and alter ankle mechanics. Procurvatum would be an ADTA less than 78°.

Correct Answer: B

Rationale: The text specifically defines the tibial mechanical axis as a line connecting the center of the tibial plateau to the center of the tibial plafond. This axis is used to measure both the LDTA and ADTA.

Correct Answer: C

Rationale: The principles of deformity correction, whether acute or gradual with an external fixator, are based on the CORA. The hinges of the fixator should be aligned with the CORA to produce the desired angular correction without inducing secondary deformities, as described by Paley's rules.

Correct Answer: C

Rationale: This is a direct application of Paley's Rule 1. When both the osteotomy and the hinge are at the CORA, the result is a pure angular correction, which is the goal of an anatomic deformity correction.

Correct Answer: C

Rationale: The text states that the mechanical axis of the lower extremity normally bisects the knee joint. When it deviates medially or laterally from the center of the knee, this is known as Mechanical Axis Deviation (MAD).

Correct Answer: C

Rationale: The normal LDTA is 86°-92°. An LDTA of 75° (<86°) indicates a varus deformity. An LDTA of 100° (>92°) indicates a valgus deformity. Therefore, the patient has a right varus and a left valgus deformity.

Correct Answer: C

Rationale: To achieve an upright posture with a hip fused in flexion, the patient must hyperextend the lumbar spine. This compensatory hyperlordosis causes repetitive crushing of the posterior elements, leading to facet arthropathy and pain. Pelvic obliquity (A) is associated with coronal plane deformities, not sagittal plane flexion.

Correct Answer: B

Rationale: A hip fused in abduction makes the ipsilateral leg functionally longer. This forces a pelvic hike and places a significant valgus stress on the knee, shifting the mechanical axis laterally and accelerating lateral compartment wear. An adduction deformity would cause functional shortening and a varus thrust (A).

Correct Answer: D

Rationale: The optimal position for a hip arthrodesis is 20° to 30° of flexion. This position provides a crucial balance, allowing the patient to stand and walk without excessive lumbar lordosis while still being able to sit down with relative ease. Less flexion makes sitting difficult, while more flexion causes severe back pain.

Correct Answer: C

Rationale: In a fused hip, the pelvis becomes the proximal segment. The deformity exists between the pelvic reference line and the femoral mechanical axis. The apex of this deformity, or the true CORA, is located at the anatomical pivot point, which is the center of the fused femoral head. The osteotomy (D) is performed distal to the CORA.

Correct Answer: D

Rationale: This scenario is governed by Paley's Osteotomy Rule 2. Because the osteotomy is performed at a level (subtrochanteric) different from the CORA (femoral head), pure angular correction is insufficient. The distal fragment must be translated to make the distal mechanical axis collinear with the ideal proximal axis, thus correcting the Mechanical Axis Deviation (MAD) at the knee.

Correct Answer: D

Rationale: A corrective osteotomy is the superior choice. A THA (A) would be catastrophically unstable due to the lack of abductor function, leading to recurrent dislocations. The patient's young age and high physical demands also make an osteotomy preferable, as it preserves bone stock and avoids issues of implant wear and loosening.

Correct Answer: E

Rationale: In an older, low-demand patient, the primary goal is to restore hip motion to unload the spine. A THA takes down the fusion, restores motion at the hip, and immediately eliminates the need for compensatory lumbar hyperextension, thereby alleviating the back pain. A corrective osteotomy (C) would improve alignment but would not restore motion, which is the key to relieving the spine in this patient population.

Correct Answer: D

Rationale: A hip fused in adduction creates a functional limb shortening and shifts the entire limb medially. This causes the mechanical axis, which originates from the femoral head, to pass medial to the knee center, creating a large medial Mechanical Axis Deviation (MAD) and a varus thrust. Abduction (C) would cause a lateral MAD.

Correct Answer: A

Rationale: A hip in abduction creates a chronic valgus thrust at the knee. Over years, according to Wolff's law, the distal femur can structurally remodel to accommodate this force. This results in a true bony valgus deformity, which is represented by a decrease in the mLDFA (normal is ~87°). An adducted hip would cause varus changes (C, E).

Correct Answer: D

Rationale: While the iliac crests (C) and ischial tuberosities can be used, the inferior sacroiliac joints or the radiographic teardrops are often the most consistent and reliable landmarks for establishing a true horizontal pelvic line, as they are less affected by developmental abnormalities like dysplasia or post-surgical changes.

Correct Answer: C

Rationale: This scenario describes a violation of Paley's Osteotomy Rule 2, leading to the situation in Rule 3. When an osteotomy is performed outside the CORA and only angulation is applied without the necessary translation, a secondary translation deformity, known as a "Z-deformity," is created. The mechanical axis will remain uncorrected.

Correct Answer: C

Rationale: For a young, high-level athlete, a THA is a poor choice due to the near certainty of accelerated polyethylene wear, aseptic loosening, and the need for multiple complex revisions over their lifetime. A corrective osteotomy provides a durable, biological solution that can withstand high-impact loading indefinitely.

Correct Answer: C

Rationale: To be "THA-friendly," the osteotomy must not create a sharp angle or "Z-deformity" in the proximal femur. Maintaining a straight, linear path for the medullary canal is paramount, as this will allow a standard straight THA stem to be passed in the future without requiring another corrective osteotomy. Using cement (B) or resecting the trochanter (E) would severely complicate a future THA.

Correct Answer: B

Rationale: The Joint Line Convergence Angle (JLCA) describes the opening of the joint line due to ligamentous laxity. In this case, the chronic varus thrust from the adducted hip has stretched the lateral collateral ligament complex, causing the joint to "open up" on the lateral side when a varus stress is applied or during weight-bearing. This is a dynamic, ligamentous deformity.

Correct Answer: B

Rationale: The "unmasking" effect occurs when correcting a primary deformity (at the hip) reveals a secondary, previously compensated deformity (at the knee) that now becomes symptomatic. The patient's original gait pattern may have protected the knee, but the new, corrected alignment now loads the knee differently, making the underlying knee problem apparent. This is why a global assessment is crucial.

Correct Answer: C

Rationale: The optimal position for hip arthrodesis includes 0° to 15° of external rotation. This slight external rotation facilitates foot clearance during the swing phase of gait and allows the legs to be positioned more comfortably when sitting, preventing the knees from knocking together. Excessive external rotation (D, E) would cause an out-toeing gait, while internal rotation (A) is poorly tolerated.

Correct Answer: C

Rationale: An angled blade plate is an excellent choice for a THA-friendly osteotomy because it achieves rigid fixation through its blade in the femoral neck and screws in the shaft, but it does not violate or fill the medullary canal. This preserves the endosteal environment, making future reaming and insertion of a THA stem much simpler compared to an intramedullary device.

Correct Answer: C

Rationale: Severe morbid obesity is a major risk factor for complications after THA, most notably periprosthetic joint infection (PJI). The risk is so significantly elevated that it often makes elective arthroplasty contraindicated. A corrective osteotomy, while still a major surgery, avoids placing a large foreign body and has a much lower risk of deep, catastrophic infection in this high-risk population.

Correct Answer: B

Rationale: A hip fused in adduction creates a functional limb shortening. To get the foot to the ground during the stance phase, the patient must drop the pelvis on the contralateral (swing phase) side. This pelvic drop is the hallmark of a Trendelenburg or lurching gait. A pelvic hike (A) is seen with a functionally long leg, as in an abduction deformity.

Correct Answer: C

Rationale: The goal for coronal plane alignment is 0° to 5° of mechanical abduction. For planning purposes, an ideal mechanical axis representing 0° of abduction is drawn. This line is, by definition, exactly perpendicular to the true horizontal line of the pelvis and originates from the center of the fused femoral head (the CORA). The angle between this ideal line and the actual femoral axis is the deformity to be corrected.

Correct Answer: D

Rationale: In a fused hip, the femur and pelvis move as a single biomechanical unit. Therefore, when analyzing the deformity of the limb relative to the body, the proximal segment is no longer the proximal femur (A); it is the entire pelvis. The deformity exists between the pelvic orientation and the distal femoral mechanical axis.

Correct Answer: B

Rationale: The mechanical axis of the lower limb is defined as the line of weight-bearing force, drawn from the center of the femoral head to the center of the ankle. The anatomic axis (A) is a line down the center of the medullary canal of a bone.

Correct Answer: C

Rationale: The Neck Shaft Angle (NSA) is the anatomic angle formed by the intersection of the femoral neck and shaft anatomic axes. The LPFA (A) and MPFA (B) are mechanical angles related to the mechanical axis of the femur.

Correct Answer: B

Rationale: In coxa vara, the greater trochanter migrates proximally relative to the femoral head, which causes the LPFA to pathologically increase above its normal range of 85-95°. A decreased LPFA (A) would be seen in coxa valga.

Correct Answer: D

Rationale: The proximal migration of the greater trochanter in coxa vara slackens the hip abductors (gluteus medius and minimus), shortening their resting length and severely compromising their mechanical efficiency. This abductor weakness is the direct cause of the Trendelenburg gait. The adductors (A) are often tight but are not the cause of the gait.

Correct Answer: C

Rationale: The CORA is a precise geometric point defined as the intersection of the proximal and distal axes of a deformed bone. Performing the osteotomy at the CORA (D) is the ideal scenario (Paley Rule 1), but the CORA itself is an independent geometric point.

Correct Answer: A

Rationale: Paley's Rule One states that cutting and hinging at the CORA results in pure angular correction, perfectly realigning the axes without any secondary shift or translation. Rule Two (B) and Rule Three (D) describe what happens when the osteotomy or hinge is moved away from the CORA.

Correct Answer: C

Rationale: Performing an osteotomy directly through the femoral neck (the typical location of the CORA in coxa vara) carries an unacceptably high risk of disrupting the blood supply to the femoral head, leading to AVN. The intertrochanteric region is biologically more robust and safer. Sciatic nerve injury (E) is a risk but not the primary reason for choosing this location over the femoral neck.

Correct Answer: B

Rationale: When a valgus osteotomy is performed distal to the CORA, the correction inherently shifts the distal mechanical axis laterally. To counteract this and realign the overall limb mechanical axis, the distal fragment must be translated medially. Lateral translation (D) would worsen the secondary deformity.

Correct Answer: C

Rationale: The maximum degree of passive hip adduction is the key predictor of soft tissue behavior. It directly assesses the tightness of the abductor muscles, which act as a tether and can lead to a postoperative abduction contracture if they are not compliant enough to accommodate the planned valgus correction.

Correct Answer: C

Rationale: The tight abductors can only accommodate 15° of the 35° valgus correction. The remaining 20° (35° - 15° = 20°) will manifest as a fixed abduction contracture, as the tight abductor "leash" prevents the femur from adducting to a neutral position relative to the pelvis. An adduction contracture (A) would be caused by tight adductors, not abductors.

Correct Answer: D

Rationale: The normal Neck Shaft Angle in a mature skeleton is typically between 125 and 135 degrees. A value below 120° is considered coxa vara. The range of 85-95° (A) corresponds to the normal Lateral Proximal Femoral Angle (LPFA).

Correct Answer: A

Rationale: The medial shift of the limb's mechanical axis caused by coxa vara increases the load transmitted through the medial compartment of the ipsilateral knee. This chronic overload accelerates wear and predisposes the patient to premature medial compartment osteoarthritis. Lateral compartment overload (B) would be expected with a valgus deformity.

Correct Answer: B

Rationale: This scenario describes Paley's Rule Two. When the cut is made away from the CORA but the hinge of correction remains at the CORA, the axes will be perfectly realigned. However, a translation (shift) is created at the osteotomy site, meaning the bone ends will not be perfectly apposed. A secondary translational deformity (C) occurs under Rule Three.

Correct Answer: C

Rationale: Coxa vara is a complex three-dimensional deformity. It is defined by a decreased NSA but is almost always accompanied by a shortened femoral neck and a greater trochanter that is relatively overgrown and has migrated proximally, leading to abductor insufficiency. An elongated neck (A) is characteristic of coxa valga.

Correct Answer: C

Rationale: Paley's Rule Three describes the situation where both the osteotomy level and the hinge are away from the CORA. This combination creates a secondary translational deformity in addition to the angular correction, which is generally undesirable. Rule One (A) and Rule Two (B) result in perfect axis alignment, with or without translation at the osteotomy site.

Correct Answer: C

Rationale: For acquired coxa vara, the muscle tissue is typically healthy and viscoelastic. The text states that the resulting abduction contracture can often be managed with aggressive physical therapy, as the healthy muscle will gradually stretch out. Surgical release (A) or revision (D) would be considered only if conservative management fails.

Correct Answer: B

Rationale: Mechanical Axis Deviation (MAD) quantifies the deviation of the weight-bearing line from the knee center. A medial deviation indicates an overall varus alignment of the limb. A lateral deviation would indicate a valgus alignment (A).