Master Orthopedic Lower Extremity Biomechanics, Gait & Deformity Correction for ABOS Board Review | Part 7

Correct Answer: D

Rationale: Patients with genu recurvatum lock their knee in hyperextension, placing immense, destructive stress on the posterior soft tissues, including the posterior capsule and ligaments, to prevent further collapse. Over time, this leads to progressive laxity, pain, and instability. The ACL (Option A) would be on slack in hyperextension.

Correct Answer: B

Rationale: A hip flexion contracture keeps the GRV anterior to the hip, requiring constant gluteal muscle activation to prevent forward trunk collapse. Without the passive energy release from the Y-ligament, the patient must rely heavily on their calf muscles (triceps surae) for an inefficient, purely muscular "push-off."

Correct Answer: D

Rationale: At heel strike, the GRV is posterior to the ankle, creating a plantarflexion moment that would cause the foot to slap down. This is controlled by the eccentric (lengthening) contraction of the dorsiflexors, primarily the tibialis anterior, which allows for a smooth transition to foot flat. The triceps surae (Option A) are plantarflexors and would be relatively quiet during this phase.

Correct Answer: B

Rationale: The vignette describes a classic scenario where a fixed adduction contracture forces the ipsilateral pelvis to hike up for the foot to reach the ground, creating the illusion of a short leg. This is an apparent LLD. While Perthes can cause a true LLD (A), the primary driver of the observed discrepancy in this context is the contracture.

Correct Answer: B

Rationale: As shown in the diagram, a fixed adduction contracture would cause the legs to cross. To prevent this and place the foot flat on the ground, the patient must elevate (hike) the ipsilateral pelvis. This action creates an apparent leg length discrepancy.

Correct Answer: D

Rationale: A standing long-cassette radiograph (scanogram or orthoroentgenogram) is the gold standard for measuring true, structural LLD. It provides osseous measurements from the femoral head to the tibial plafond, which are immune to the confounding effects of pelvic tilt or joint contractures. The block test (C) measures the functional or apparent discrepancy, not the true structural one.

Correct Answer: A

Rationale: At heel strike, the GRV is normally anterior to the hip, creating a flexion moment that must be opposed by the hip extensors (gluteus maximus). By lurching backward, the patient shifts their center of mass and the GRV posterior to the hip joint, creating a passive extension moment and eliminating the need for strong hip extensor contraction.

Correct Answer: B

Rationale: The posterior position of the GRV relative to the knee during loading response creates a powerful flexion moment. The quadriceps must contract eccentrically to control this knee flexion, absorbing the shock of weight acceptance. Hamstrings (A) are knee flexors and would exacerbate the instability.

Correct Answer: C

Rationale: In the face of pelvic obliquity from a true LLD, the lumbar spine develops a compensatory curve. This adaptation is crucial to bring the torso and head back to a vertical, centered orientation over the pelvis, thereby maintaining global balance and a level line of sight.

Correct Answer: C

Rationale: This is a critical error. The patient's limbs were already of equal anatomical length. The "shortening" was apparent, caused by the contracture. By lengthening the bone, the surgeon creates a true structural LLD, making the limb anatomically longer and severely worsening the patient's gait and overall biomechanics.

Correct Answer: E

Rationale: In mid-stance, the gastrocsoleus complex eccentrically contracts to control forward tibial progression over the foot. This action keeps the knee joint center posterior to the ground reaction vector, which in turn creates a passive external extension moment, stabilizing the knee without requiring quadriceps contraction.

Correct Answer: D

Rationale: At heel strike, the GRV is posterior to the ankle, creating a plantar flexion moment. The tibialis anterior and other dorsiflexors (innervated by the deep peroneal nerve) must contract eccentrically to control this, allowing the foot to gently lower to the ground. Paralysis of these muscles results in an uncontrolled, slapping motion.

Correct Answer: B

Rationale: Initially, compensatory scoliosis is flexible. Over decades of asymmetric loading, permanent changes occur, including vertebral disc wedging and facet arthropathy. This causes the curve to become rigid and structural, meaning it will no longer correct when the patient sits or the LLD is addressed.

Correct Answer: B

Rationale: The most demanding phase for the quadriceps is the loading response, when the GRV is posterior to the knee, creating a flexion moment. By forcing the knee into hyperextension, the patient shifts the GRV anterior to the knee joint's center of rotation. This creates an external extension moment, which stabilizes the knee without requiring quadriceps contraction.

Correct Answer: B

Rationale: The block test is a clinical maneuver that quantifies the functional deficit caused by pelvic obliquity, whether from a true LLD or a joint contracture. It measures the amount of lift needed to make the pelvis level during standing, which represents the apparent LLD. It does not measure the true osseous LLD (A), which requires radiography.

Correct Answer: C

Rationale: The text emphasizes that the true goal of deformity correction extends beyond static radiographic parameters. The ultimate success is measured by the patient's functional outcome, specifically the restoration of normal dynamic biomechanics, which manifests as an efficient and painless gait.

Correct Answer: C

Rationale: Given the clinical picture of an apparent LLD due to a hip contracture, the most critical step is to establish the ground truth of the skeletal anatomy. A standing long-cassette radiograph (scanogram) will definitively measure the true, structural limb lengths, preventing the catastrophic error of lengthening a limb that is not actually short.

Correct Answer: B

Rationale: At the moment of heel strike, the GRV passes posterior to the ankle joint center, creating a plantar flexion moment. Simultaneously, it passes anterior to the hip joint center, creating a flexion moment.

Correct Answer: D

Rationale: Sitting eliminates the effect of the lower extremity length discrepancy, which levels the pelvis. Since the scoliosis is a flexible compensation for the pelvic obliquity, it will correct and disappear when the pelvis is level. This is a key feature distinguishing a flexible from a structural scoliosis.

Correct Answer: D

Rationale: Measuring from a non-fixed point like the umbilicus makes the measurement highly dependent on the position of the pelvis. Any pelvic obliquity (tilt), whether from a contracture or true LLD, will alter the distance from the umbilicus to the malleolus, making this method a measure of apparent, not true, LLD.

Correct Answer: B

Rationale: By restraining the tibia, the calf muscles ensure the ground reaction vector passes anterior to the knee joint center. A force vector anterior to a joint's center of rotation creates an external extension moment, which passively stabilizes the knee in extension.

Correct Answer: B

Rationale: A fixed flexion deformity (FFD) at the knee prevents the limb from fully extending, making its functional length shorter. Since the bones themselves are of equal length, this is a classic example of an apparent LLD caused by a soft-tissue contracture, similar in principle to a hip adduction contracture.

Correct Answer: B

Rationale: The introductory text explicitly states that mastery requires understanding the intricate dance between the Ground Reaction Vector (GRV), the Mechanical Axis Deviation (MAD), and the Center of Rotation of Angulation (CORA) to plan and execute corrective osteotomies.

Correct Answer: A

Rationale: The Mechanical Axis Deviation (MAD) is the perpendicular distance from the center of a joint (in this case, the knee) to the mechanical axis of the limb (femoral head to ankle center). A medial deviation indicates a varus deformity. The LDTA and MPTA are joint orientation angles, not global alignment measurements.

Correct Answer: A

Rationale: The normal range for the LDTA is 86° to 92°. A value greater than 92°, such as 95°, indicates a valgus orientation of the distal tibial articular surface. A value less than 86° would indicate varus.

Correct Answer: A

Rationale: The normal ADTA is 78° to 82°. A decreased ADTA (apex posterior deformity) limits passive ankle dorsiflexion, which is the essential motion for the second (ankle) rocker, where the tibia advances over the fixed foot. This blockade prevents smooth forward progression during midstance.

Correct Answer: A

Rationale: The Center of Rotation of Angulation (CORA) is the geometric apex of a deformity, precisely located at the intersection of the proximal and distal mechanical axes of the deformed bone. This is the fundamental starting point for planning a corrective osteotomy.

Correct Answer: A

Rationale: Paley's Rule 1 states that when the osteotomy and the axis of correction (hinge) both pass through the CORA, pure angular correction is achieved without any translation of the bone segments. This is the most geometrically perfect correction.

Correct Answer: A

Rationale: Anterior translation increases the length of the anterior lever arm. During the swing phase, the anterior compartment muscles (e.g., tibialis anterior) must work exponentially harder to dorsiflex the ankle and clear this longer segment from the ground, leading to fatigue and a compensatory steppage gait.

Correct Answer: A

Rationale: The anteriorly translated foot creates a mechanical block to the second (ankle) rocker. The body cannot smoothly advance the tibia over the foot. Premature heel rise is the body's inefficient attempt to get over this mechanical obstruction and initiate the swing phase.

Correct Answer: A

Rationale: The graph shows a dramatically shortened stance time on the affected (red) limb. Because this limb cannot provide support for the normal duration, the contralateral (normal) limb must make ground contact prematurely to "catch" the body. This results in a shortened step length on the normal side, creating an asymmetric, limping gait.

Correct Answer: A

Rationale: By externally rotating the leg, the patient changes the axis of progression from a long sagittal path to a shorter, oblique path over the medial border of the foot. This is a clever but damaging compensation to bypass the mechanical block of a long anterior lever arm.

Correct Answer: A

Rationale: The second (ankle) rocker requires the tibia to glide anteriorly over the talus. When this is blocked by fusion and/or anterior translation, the body's forward momentum thrusts the tibia backward. Genu recurvatum is a vaulting mechanism to keep the center of mass moving forward over the rigid ankle-foot complex.

Correct Answer: A

Rationale: The third (forefoot) rocker, powered by a concentric contraction of the triceps surae, provides the primary "push-off" or propulsive force for gait. When this is lost, the patient must use proximal muscles (core, paraspinals) to rotate the pelvis and drag the limb forward.

Correct Answer: A

Rationale: Posteriorly translating the foot relative to the tibial mechanical axis is critical. This shortens the anterior lever arm, facilitating a smooth rollover and mitigating the mechanical block caused by the fusion. Anterior translation is a devastating iatrogenic error.

Correct Answer: A

Rationale: Posterior translation of the foot shifts the ground reaction force (GRF) vector anterior to the center of the knee joint. This creates a natural extension moment at the knee, which passively stabilizes it and reduces the active firing required from the quadriceps muscle, thus lowering the metabolic cost of walking.

Correct Answer: A

Rationale: Fusing the ankle in 5° to 10° of external rotation (matching the contralateral side) causes the forefoot rocker to occur along a shorter, oblique path across the metatarsal heads. This synergizes with posterior translation to make the rollover mechanism even more efficient.

Correct Answer: A

Rationale: Achieving slight hindfoot valgus recreates the normal loading pattern of the hindfoot. It positions the mechanical axis of the calcaneus slightly lateral to the tibial mechanical axis, which provides a stable base of support and prevents a varus thrust during gait.

Correct Answer: A

Rationale: Fusing an ankle in varus is a catastrophic error. It concentrates forces on the lateral border of the foot, leading to pain, instability, stress fractures of the fifth metatarsal, and a highly dysfunctional gait. Valgus, posterior translation, neutral dorsiflexion, and external rotation are all desirable goals.

Correct Answer: A

Rationale: Paley's Rules 2 and 3 describe situations where the osteotomy is not performed at the level of the CORA. This separation of the osteotomy site from the CORA results in both angular correction and a predictable translation of the bone segments, which must be calculated and planned for.

Correct Answer: A

Rationale: Isolated ankle films are insufficient for comprehensive deformity planning. A full-length radiograph is mandatory to assess the global limb alignment by calculating the MAD and to identify any compensatory deformities in the proximal tibia or femur that must be addressed.

Correct Answer: A

Rationale: The first rocker begins at initial contact (heel strike) and involves the heel acting as a fulcrum to smoothly lower the foot to the floor. This action is controlled by the eccentric work of the dorsiflexors (e.g., tibialis anterior) and serves as a shock absorber.

Correct Answer: A

Rationale: The third rocker is the propulsive phase of gait. It requires the foot to transform into a rigid lever (via the windlass mechanism) so that the powerful concentric contraction of the triceps surae can effectively push off from the metatarsal heads.

Correct Answer: A

Rationale: With the second rocker blocked by the anterior translation, the patient is unable to transfer weight from the hindfoot to the forefoot. The entire stance phase is spent loading the heel, which bears the brunt of the ground reaction forces, as confirmed by pedobarography.

Correct Answer: C

Rationale: The fundamental goal of an HTO for medial compartment arthritis is to correct the varus deformity by shifting the mechanical axis from the medial compartment to a slightly valgus position (Fujisawa point), thereby unloading the diseased medial cartilage. Shifting the axis medially (A) would worsen the condition.

Correct Answer: B

Rationale: A valgus deformity originating in the distal femur is characterized by an abnormal mechanical Lateral Distal Femoral Angle (mLDFA). A normal mLDFA is approximately 85-90 degrees; in a valgus deformity, this angle would be decreased (e.g., <85 degrees). The MPTA (A) relates to tibial alignment.

Correct Answer: D

Rationale: A varus deformity originating in the proximal tibia is defined by an abnormal Medial Proximal Tibial Angle (MPTA). A normal MPTA is 85-90 degrees. In a varus deformity, this angle is decreased (e.g., <85 degrees). While MAD (C) indicates overall limb malalignment, MPTA localizes the deformity to the proximal tibia.

Correct Answer: B

Rationale: The Center of Rotation of Angulation (CORA) is the geometric point representing the apex of a deformity, defined by the intersection of the proximal and distal axes of the deformed bone. The ACA (A) is the axis around which the correction is performed, which may or may not be at the CORA.

Correct Answer: D

Rationale: The Longitudinal Bisector Line (lBL) is the line that passes through the CORA and bisects the acute angle of the deformity. The Transverse Bisector Line (tBL) (C) is perpendicular to it and bisects the obtuse angle.

Correct Answer: B

Rationale: The Transverse Bisector Line (tBL) is defined as the line passing through the CORA that bisects the obtuse angle of the intersecting axes. It is, by definition, perpendicular to the Longitudinal Bisector Line (lBL).

Correct Answer: C

Rationale: The Angulation Correction Axis (ACA) is the dynamic, mechanical axis around which the surgeon physically rotates the bone segments to correct the deformity. The CORA (A) is a static, geometric point that defines the location of the deformity's apex.

Correct Answer: B

Rationale: The geometric model for predicting translation and length changes is centered on the Angulation Correction Axis (ACA). The orthogonal axes of this graph (tBL' and lBL') are parallel to the original tBL and lBL but are centered on the ACA, not the CORA.

Correct Answer: B

Rationale: The lBL' divides the bone into a convex and concave side relative to the ACA. Points on the convex side are distracted (lengthened), while points on the concave side are compressed (shortened) during the angular correction.

Correct Answer: C

Rationale: The defining characteristic of an opening wedge osteotomy is the placement of the ACA at the convex cortex (in this case, the lateral cortex of the tibia). This point acts as the hinge, causing the concave (medial) side to distract and open.

Correct Answer: C

Rationale: An opening wedge osteotomy inherently lengthens the bone because the correction occurs via distraction on the concave side while hinging on the convex cortex. This is advantageous in patients with a preexisting shortening on the affected side.

Correct Answer: D

Rationale: The common peroneal nerve is relatively fixed as it courses around the fibular neck. A large opening of the medial side of the proximal tibia places this nerve under significant tension, creating a risk for a traction neuropraxia.

Correct Answer: C

Rationale: This scenario describes Paley's Rule 2 (Angulation-Translation). When the ACA is at the CORA but the osteotomy is at a different level, the overall mechanical axis is perfectly realigned, but this requires the bone ends at the cut site to translate relative to each other.

Correct Answer: B

Rationale: A closing wedge osteotomy inherently shortens the bone. In a patient with a valgus deformity and a longer affected leg, a medial closing wedge distal femoral osteotomy corrects the angulation while advantageously addressing the leg length discrepancy.

Correct Answer: B

Rationale: The Mechanical Axis of the Lower Extremity (MALE) is defined as the straight line connecting the center of the femoral head to the center of the ankle plafond. The Anatomic Axis follows the mid-diaphyseal line of the bones, which is different, especially in the femur.

Correct Answer: C

Rationale: The text explicitly states that unlike the femur, the mechanical and anatomic axes of the tibia are nearly parallel, usually within 0° to 2° of each other. This unique feature makes anatomic axis planning a reliable method for tibial diaphyseal deformities.

Correct Answer: B

Rationale: The standard population average for the mLDFA is 87°, with a normal range of 85°–90°. This angle is critical for assessing the orientation of the distal femur relative to its mechanical axis.

Correct Answer: B

Rationale: The standard population average for the MPTA is 87°, with a normal range of 85°–90°. Restoring this angle is a primary goal of a high tibial osteotomy for varus deformity.

Correct Answer: C

Rationale: The normal LDTA is 90°, with a range of 88°–92°. This indicates that the tibial plafond is typically perpendicular to the mechanical axis of the tibia.

Correct Answer: C

Rationale: An abnormal JLCA (normally 0-2°) indicates a soft-tissue component to the deformity, such as ligamentous laxity (e.g., LCL stretching in varus) or severe cartilage loss in one compartment. Ignoring this can lead to under- or over-correction.

Correct Answer: B

Rationale: The text highlights that the near-parallel relationship (0-2° difference) between the tibial mechanical and anatomic axes is a unique anatomical quirk. This makes Anatomic Axis Planning a practical and accurate alternative to Mechanical Axis Planning, especially for diaphyseal deformities.

Correct Answer: C

Rationale: The text emphasizes that the goal is not just to make a bone straight, but to restore normal joint orientation. This involves correcting angles like the MPTA and LDTA to ensure proper load distribution across the knee and ankle joints, thereby preserving them.

Correct Answer: D

Rationale: The Malalignment Test requires a global assessment of the entire lower extremity's weight-bearing axis. This can only be achieved with a weight-bearing, full-length standing AP radiograph that includes the hip, knee, and ankle joints.

Correct Answer: C

Rationale: The text explicitly states that failure to control for rotation (the "patella forward" view) is the most common point of failure. It invalidates measurements by projecting a frontal plane deformity into an oblique plane, confounding the analysis.

Correct Answer: C

Rationale: In a normal, healthy limb, the MALE passes slightly medial to the exact center of the knee joint, approximately 8mm medial to the center (Kennedy Zone 1). This creates a slight physiological varus alignment.

Correct Answer: B

Rationale: When the MALE passes significantly medial to the knee center, it is defined as Medial Mechanical Axis Deviation (MAD). This corresponds to a varus alignment of the lower limb.

Correct Answer: B

Rationale: If the MALE passes lateral to the knee center, the patient has Lateral Mechanical Axis Deviation (MAD), which is characteristic of a valgus alignment of the lower extremity.

Correct Answer: C

Rationale: The protocol dictates that the first step in tibial mechanical axis planning is to establish the true mechanical axis of the proximal segment, known as the Proximal Mechanical Axis (PMA).

Correct Answer: B

Rationale: In the "Normal Scenario" where the femur and knee joint are normally aligned (normal mLDFA and JLCA), the PMA of the tibia is simply a distal extension of the femoral mechanical axis straight across the knee joint.

Correct Answer: C

Rationale: When the ipsilateral femur is also deformed, you cannot use it as a reference. The text describes the "Abnormal Scenario" where you must reconstruct the PMA by drawing a line from the knee center at the ideal MPTA (87°) relative to the tibial plateau's joint line.

Correct Answer: B

Rationale: The DMA is drawn by finding the center of the ankle joint and extending a line proximally into the shaft, keeping it parallel to the straight, undeformed portion of the distal tibial diaphysis.

Correct Answer: C

Rationale: The Malorientation Test (MOT) specifically evaluates the orientation of the distal segment relative to the ankle. An abnormal LDTA found during the MOT indicates a second deformity near the ankle, which is critical for surgical planning.

Correct Answer: C

Rationale: An abnormal LDTA (anything significantly different from 90°) during the MOT is a critical finding. It proves that the deformity is not simple and that a second apex of deformity exists near the ankle, which will likely require a separate distal osteotomy.

Correct Answer: A

Rationale: The CORA is the geometric apex of the deformity. It is the point around which the bone has angulated. Identifying the CORA is the central goal of planning, as it dictates the ideal location for the osteotomy and hinge of correction.

Correct Answer: B

Rationale: The mechanical axis of the lower extremity is a straight line connecting the center of the femoral head to the center of the ankle. It represents the line of weight-bearing force transmission. The anatomic axis follows the intramedullary canal of the femur and tibia, which is not a straight line for the entire limb.

Correct Answer: A

Rationale: The Center of Rotation of Angulation (CORA) is the point of intersection of the proximal and distal axes of a deformed bone. Placing an osteotomy at the CORA allows for correction of the angulation without introducing translation. The Apex of Deformity (AOD) is a less precise term for the general location of the deformity's peak.

Correct Answer: C

Rationale: Mechanical Axis Deviation (MAD) is the perpendicular distance from the center of the knee to the mechanical axis of the limb. A medial deviation, as seen here, indicates a varus deformity, while a lateral deviation indicates a valgus deformity. MPTA and LDFA are joint orientation angles, not measures of overall limb deviation.

Correct Answer: B

Rationale: The clinical and radiographic description corresponds to genu valgum, or "knock-knees," where the mechanical axis is deviated laterally. This leads to increased compressive forces on the lateral compartment of the knee. Genu varum is the opposite deformity, with medial deviation of the mechanical axis.

Correct Answer: C

Rationale: The combination of progressive, severe genu varum in a young child with radiographic findings of medial metaphyseal beaking is characteristic of Blount's disease, a pathologic condition of the medial proximal tibial physis. Physiologic genu varum is typically milder and improves by age 2-3.

Correct Answer: B

Rationale: The Medial Proximal Tibial Angle (MPTA) measures the orientation of the proximal tibial joint surface relative to the axis of the tibia. It can be measured relative to the anatomic (aMPTA) or mechanical axis (mMPTA). A value less than the normal of ~87 degrees indicates a varus deformity originating from the tibia.

Correct Answer: C

Rationale: The radiograph demonstrates multiple points of angulation within the same limb segment (femur and tibia). This is defined as a multi-apical deformity. Correcting such a deformity often requires multiple osteotomies, each planned at a specific CORA, to restore normal alignment without creating secondary deformities.

Correct Answer: A

Rationale: A windswept deformity is characterized by one lower extremity having a valgus deformity while the contralateral limb has a varus deformity, giving the appearance that the legs have been "swept" to one side. This is often seen in patients with metabolic bone disease, skeletal dysplasias, or certain neuromuscular conditions.

Correct Answer: C

Rationale: The Joint Line Convergence Angle (JLCA) reflects the "gapping" of the joint. It is not a measure of bony deformity but rather of soft tissue factors, including ligamentous laxity and asymmetric cartilage loss. A normal JLCA is 0-2 degrees. An increased JLCA in a varus knee, for example, indicates lateral ligamentous laxity or medial cartilage collapse.

Correct Answer: C

Rationale: Fibular hemimelia is a congenital longitudinal deficiency characterized by the partial or complete absence of the fibula. It is classically associated with limb length discrepancy, anterior bowing of the tibia (procurvatum), and complex foot and ankle deformities, including equinovalgus or equinovarus.

Correct Answer: C

Rationale: The radiograph shows significant depression and fragmentation of the medial tibial metaphysis, which corresponds to Langenskiöld Stage IV. This stage is characterized by a step-off in the articular surface and indicates a severe form of the disease with a high likelihood of progression and the formation of a physeal bar across the medial physis, which would tether growth.

Correct Answer: C

Rationale: A varus alignment shifts the mechanical axis medially, increasing the lever arm for the ground reaction force. This creates a larger external adduction moment at the knee, which must be balanced by internal forces. This increased adduction moment leads to elevated compressive forces on the medial compartment and tensile forces on the lateral collateral ligament (LCL), not the MCL.

Correct Answer: A

Rationale: In a patient with genu valgum, the deformity is often located in the distal femur (abnormal mLDFA). To correct this, a distal femoral osteotomy is performed to create a varus alignment in the bone itself. This realigns the limb, shifting the overall mechanical axis from a lateral (valgus) position back to the center of the knee.

Correct Answer: C

Rationale: The fundamental principle of the CORA is that it is the geometric center of the deformity. By rotating the distal fragment around a hinge placed at the CORA, the surgeon can correct the angulation perfectly without causing the bone fragments to shift or translate relative to each other, thus restoring the axis in the most anatomically correct way.

Correct Answer: D

Rationale: Genu varum is physiologic in children up to 2-3 years of age. A metaphyseal-diaphyseal angle less than 11 degrees is highly suggestive of physiologic bowing, which typically resolves spontaneously. Given the patient's age and the likely radiographic finding, observation is the correct course of action. Surgical intervention is reserved for pathologic conditions like Blount's disease or persistent deformity.

Correct Answer: C

Rationale: A normal, or neutrally aligned, lower limb has a slight physiologic varus. This means the mechanical axis passes just medial to the geometric center of the knee, typically through the medial tibial spine. This results in slightly higher compressive loads on the medial compartment compared to the lateral compartment during normal gait.

Correct Answer: B

Rationale: Genu valgum shifts the mechanical axis laterally, increasing the compressive load on the lateral compartment of the knee. Over time, this leads to accelerated wear of the lateral femoral condyle and lateral tibial plateau cartilage, as well as degeneration of the lateral meniscus. The medial collateral ligament (MCL) would be under increased tensile stress.

Correct Answer: C

Rationale: In a multi-apical deformity, correcting the entire limb malalignment with a single osteotomy at one level would realign the mechanical axis but would create an abnormal orientation of the knee joint (i.e., an oblique joint line). By addressing each deformity at its respective CORA in the femur and tibia, the surgeon can restore both the overall mechanical axis and the normal, near-perpendicular orientation of the knee joint to that axis.

Correct Answer: C

Rationale: The underlying cause of the bony deformities in hypophosphatemic rickets is a metabolic defect in phosphate regulation. While surgery is often necessary to correct severe deformities, it is crucial to first optimize the patient's metabolic status with medical management (oral phosphate and calcitriol). Surgery in a metabolically active state can lead to poor healing and rapid recurrence of the deformity.

Correct Answer: B

Rationale: The normal mechanical lateral distal femoral angle (mLDFA) is 87° ± 3° (range, 85°-90°). This angle is crucial for assessing frontal plane alignment of the femur. An angle greater than 90° indicates valgus, while an angle less than 85° indicates varus. 81° is the approximate normal value for the anatomic lateral distal femoral angle (aLDFA), not the mechanical angle.

Correct Answer: C

Rationale: The normal medial proximal tibial angle (MPTA) is 87° ± 3° (range, 85°-90°). This value represents a slight varus orientation of the proximal tibia relative to its mechanical axis. An MPTA less than 85° indicates tibia vara. 81° is the normal value for the posterior proximal tibial angle (PPTA) in the sagittal plane.

Correct Answer: D

Rationale: In a normally aligned lower limb, the mechanical axis passes slightly medial to the center of the knee, typically within a range of 0 to 10 mm. This creates a slight varus mechanical alignment, which is considered physiologic. A line passing lateral to the knee center would indicate a valgus mechanical alignment.