Orthopedic Board Review Questions: ABOS Part I, AAOS OITE, Paley Method, Varus Knee, LCL Laxity | Part 22010

Correct Answer: D

The Mechanical Axis Deviation (MAD) is the foundational measurement for quantifying the overall magnitude of lower limb alignment. As stated in the text, 'The Mechanical Axis Deviation (MAD) is the foundational measurement of lower limb alignment. It represents the absolute magnitude of the deformity.' It is defined as the perpendicular distance from the exact center of the knee joint to the mechanical axis line. While mLDFA and MPTA are critical for pinpointing the anatomical source of the deformity (femur or tibia), and JLCA is paramount for assessing intra-articular ligamentous laxity, MAD provides the initial, comprehensive assessment of how far the mechanical axis deviates from the ideal center of the knee, thus quantifying the total angular correction required. The Anatomical Tibial Angle (ATA) is not mentioned as a primary measurement for overall alignment in the provided text.

Correct Answer: C

The text explicitly states, 'The JLCA (Joint Line Convergence Angle) is the paramount angle for assessing ligamentous laxity and intra-articular deformity.' It further clarifies, 'In a knee with significant LCL laxity, the lateral compartment will 'open up' on a weight-bearing radiograph, resulting in an abnormally high JLCA (e.g., 5-10° open laterally).' The image provided likely illustrates this concept, showing the joint line opening. A decreased mLDFA or increased MPTA would indicate a bony deformity in the femur or tibia, respectively, but not directly ligamentous laxity. A medial shift of the MAD quantifies the overall varus, which can be due to bone or soft tissue, but doesn't specifically pinpoint ligamentous laxity. Posterior tibial slope is not discussed in the context of LCL laxity in the provided text.

Correct Answer: B

The text clearly outlines the cornerstone concept: 'To effectively neutralize the powerful adduction moment and recenter the load over the tibial plateau in a ligamentously deficient knee, the surgeon must intentionally overcorrect the bony alignment.' It further states, 'The primary goal is to shift the mechanical axis from the diseased medial compartment, across the center of the knee, and into the healthy lateral compartment. For a varus knee with severe LCL laxity, this means overcorrecting the limb into at least 3 to 5 degrees of mechanical tibiofemoral valgus.' This overcorrection functionally compensates for the ligamentous instability by creating a stable bony alignment that prevents the joint from thrusting open. Overcorrection does not directly tighten the LCL, nor is its primary goal to reduce limb length discrepancy, prevent meniscal extrusion, or improve patellofemoral tracking in this context.

Correct Answer: B

The text defines the normal values for joint orientation angles: 'mLDFA (Mechanical Lateral Distal Femoral Angle): Normal value is 87° (range 85-90°).' and 'MPTA (Mechanical Proximal Tibial Angle): Normal value is 87° (range 85-90°).' In this scenario, the mLDFA is 87°, which is within the normal range, indicating no significant deformity in the distal femur. However, the MPTA is 75°, which is significantly less than the normal 87°. A decreased MPTA indicates a varus deformity originating in the proximal tibia. While the image shows a varus deformity, the specific measurements provided pinpoint the proximal tibia as the primary bony source. Intra-articular deformity would be indicated by an abnormal JLCA, which is not directly given here, though it might be present secondarily.

Correct Answer: B

The text explicitly details the effects of chronic medial overloading in genu varum: 'To counteract the massive adduction moment, the lateral structures—specifically the LCL, the popliteus tendon, the biceps femoris, and the posterolateral capsule—are placed under constant, abnormal tensile stress. Over months and years of thousands of daily gait cycles, these collagenous tissues undergo plastic deformation. They stretch, thin, and lose their elastic modulus, a process clinically referred to as 'soft tissue attenuation.'' This directly corresponds to option B. Options A, C, and D describe effects contrary to what is stated in the text. While increased stiffness might occur in some pathological processes, the text specifically highlights 'plastic deformation,' 'stretch,' 'thin,' and 'lose their elastic modulus,' which implies attenuation rather than increased stiffness.

Correct Answer: C

The text defines the CORA: 'The Center of Rotation of Angulation (CORA) is the geometric apex of a bony deformity. It is found at the intersection of the proximal and distal mechanical (or anatomical) axis lines of a deformed bone.' The image provided, depicting intersecting lines representing mechanical axes, is a classic illustration of how the CORA is identified. The other options represent different measurements or concepts: MAD is the overall deviation, JLCA is for joint line convergence/divergence, MPTA is a joint orientation angle, and the point of maximum cartilage wear is a clinical finding, not a geometric point defined by intersecting axes.

Correct Answer: C

This patient presents with both a significant bony varus deformity (20mm medial MAD) and clear evidence of ligamentous laxity (7° laterally open JLCA and varus thrust). The text emphasizes that 'Addressing the bony deformity alone in the face of significant ligamentous laxity is a recipe for catastrophic surgical failure.' It advocates for intentional overcorrection: 'To effectively neutralize the powerful adduction moment and recenter the load over the tibial plateau in a ligamentously deficient knee, the surgeon must intentionally overcorrect the bony alignment... For a varus knee with severe LCL laxity, this means overcorrecting the limb into at least 3 to 5 degrees of mechanical tibiofemoral valgus.' Therefore, performing a proximal tibial osteotomy (assuming the tibia is the primary bony deformity, which is common in varus) to achieve 3-5 degrees of mechanical tibiofemoral valgus is the most appropriate strategy. A simple correction to neutral (Option A) would leave the attenuated ligaments slack. Isolated LCL reconstruction (Option B) without addressing the underlying bony malalignment would likely fail due to persistent asymmetric loading. A distal femoral osteotomy (Option D) would only be appropriate if the primary bony deformity was femoral, and even then, overcorrection would be needed. Non-operative management (Option E) is unlikely to be effective for severe, symptomatic malalignment with ligamentous laxity.

Correct Answer: B

The text explicitly describes the vicious cycle: 'Asymmetric loading across the joint accelerates cartilage wear and meniscal degradation, which in turn leads to progressive attenuation of the supporting soft tissue envelope. As the soft tissues stretch and fail, the deformity worsens, further concentrating the destructive forces on the already compromised compartment.' Option B directly mirrors this sequence: Asymmetric loading → Cartilage wear/meniscal degradation → Soft tissue attenuation → Deformity worsens. The other options present incorrect or incomplete sequences of this specific cycle as described in the text.

Correct Answer: C

Let's break down the measurements based on the text:

• MAD (15mm medial): Confirms a varus deformity.
• mLDFA (88°): This is within the normal range (85-90°), ruling out a primary distal femoral deformity.
• JLCA (1° medial convergence): This is within the normal range (0-2° convergence), indicating no significant intra-articular ligamentous laxity or joint space opening.
• MPTA (78°): This is significantly less than the normal 87° (85-90°). A decreased MPTA indicates a varus deformity originating in the proximal tibia.

Therefore, the most likely primary cause of the varus malalignment is a proximal tibial varus deformity. LCL laxity (Option A) is ruled out by the normal JLCA. Distal femoral varus (Option B) is ruled out by the normal mLDFA. Combined deformity (Option D) is unlikely as only the MPTA is abnormal. Medial meniscal extrusion (Option E) is a consequence, not the primary cause of the bony malalignment itself, and would typically be associated with a more open JLCA if it were contributing significantly to the overall alignment.

Correct Answer: C

The text explains the purpose of the secondary compensatory deformity (bony overcorrection): 'By doing so, the surgeon effectively 'masks' the ligamentous instability with a new, highly stable bony alignment that physically prevents the joint from thrusting open.' This directly aligns with option C, which states the outcome is to 'physically prevent the joint from thrusting open and provide dynamic joint stability.' The overcorrection does not directly repair the LCL (Option A), nor does it aim for a perfectly neutral mechanical axis (Option B) – in fact, it intentionally creates a valgus mechanical axis. Increasing range of motion (Option D) and reducing infection risk (Option E) are not the primary functional outcomes of this specific biomechanical strategy for addressing ligamentous laxity.

Correct Answer: C

The clinical presentation of a 'visible and sudden shift of his knee into varus during the stance phase of gait' is the classic description of a lateral thrust. This sign, combined with significant lateral joint line opening on varus stress testing at 30° of flexion, is pathognomonic for dynamic coronal plane instability primarily due to Lateral Collateral Ligament (LCL) insufficiency. The LCL is the primary static restraint against varus stress, and its incompetence allows the lateral compartment to abnormally gap open under load.

Option A is incorrect because while medial compartment osteoarthritis can be present, the dynamic lateral thrust and LCL laxity indicate a more complex instability that is not simply compensatory. The LCL insufficiency is a primary driver of the instability.

Option B is incorrect because while a fixed bony varus deformity can contribute, the description emphasizes a dynamic shift and 'giving way' sensation, which points to ligamentous instability rather than solely a fixed bony malalignment. The case highlights that bony malalignment is easily measured on static radiographs, but dynamic instability is often hidden.

Option D is incorrect because an ACL deficiency primarily causes anteroposterior instability, not typically a sudden varus thrust during gait. While multi-ligamentous injuries can occur, the specific description points to coronal plane instability.

Option E is incorrect because a severe MCL tear would lead to valgus instability and opening of the medial joint line, not a varus thrust or lateral joint line opening. The findings are opposite to what would be expected with MCL pathology.

Correct Answer: C

The case explicitly states, 'The Joint Line Congruency Angle (JLCA) becomes the most powerful diagnostic tool in the surgeon's arsenal. The JLCA measures the angle formed between the distal femoral joint line and the proximal tibial joint line... A JLCA greater than 2° is strictly pathological. It directly and mathematically quantifies the contribution of soft tissue laxity to the overall limb deformity.'

Option A is incorrect because a JLCA of 0-2° of medial convergence is described as the normal range for a healthy knee, where the femoral and tibial joint lines are nearly parallel.

Option B is incorrect because the JLCA measures the angle between the distal femoral joint line and the proximal tibial joint line, not the mechanical axes. The mechanical axis deviation (MAD) relates to the mechanical axes.

Option D is incorrect because while mLDFA and MPTA are used to determine the location of bony deformity, the JLCA is specifically highlighted as the most powerful tool for quantifying soft tissue laxity.

Option E is incorrect because the case emphasizes that 'The CORA must always be planned based on the anticipated, corrected soft-tissue anatomy.' This means the surgeon must manually 'close' the pathologically open lateral joint space (i.e., normalize the JLCA) on a tracing or digital template before calculating the CORA. Failure to do so results in inaccurate bony correction.

Correct Answer: B

The case explicitly states, 'In a patient suffering from LCL laxity, the MAD is not a static number; it is a dynamic variable that changes based on load.' It further clarifies that 'On a standard anteroposterior (AP) radiograph with the patient bearing weight equally on both legs, the MAD may appear completely normal or only mildly deviated... When the patient is instructed to stand solely on the affected limb, the LCL is fully challenged by the body's center of gravity. The lateral joint space gaps open, the knee thrusts into varus, and the MAD shifts dramatically medially, revealing the true, devastating extent of the functional malalignment.' This dynamic shift, coupled with an increased JLCA, is a direct indicator of LCL laxity. The implication for surgical planning is that both the bony deformity and the soft tissue laxity must be addressed.

Option A is incorrect because the dynamic changes in MAD and JLCA clearly indicate a significant soft tissue component (LCL laxity) that cannot be ignored. Relying solely on the double-leg stance film would lead to an incomplete and likely failed correction.

Option C is incorrect because the case emphasizes, 'single-leg stance radiographs are never optional. They are absolutely essential for unmasking the dynamic soft-tissue component of the deformity.'

Option D is incorrect because an increased JLCA (medial convergence) and a medial shift of the MAD on single-leg stance are characteristic of LCL laxity causing varus instability, not a fixed valgus deformity of the distal femur.

Option E is incorrect because a medial shift of the MAD and varus thrust are associated with lateral compartment instability (LCL), not MCL injury, which would typically lead to valgus instability and a lateral shift of the MAD.

Correct Answer: C

The case explicitly warns against this pitfall: 'When dealing with LCL laxity, the most critical mistake a surgeon can make is to calculate the CORA from a standard, unadjusted radiograph that shows a pathologically open lateral joint space. The CORA must always be planned based on the anticipated, corrected soft-tissue anatomy.' The text further clarifies, 'The actual bony osteotomy is then planned on a radiographic tracing (or digital template) where the JLCA has been manually 'closed' by the surgeon, representing the post-stabilization state of the knee. Failure to simulate this closed joint line prior to calculating the CORA will result in an inaccurate bony correction, leaving the patient with residual malalignment.'

Option A is incorrect because the method described is specifically identified as a critical mistake that leads to inaccurate correction.

Option B is incorrect because calculating the CORA from an unadjusted radiograph with an open lateral joint space would likely lead to an undercorrection of the varus bony deformity relative to the true bony deformity once the soft tissues are tightened, not an overcorrection into valgus.

Option D is incorrect because the issue is with the bony correction itself being inaccurate, not necessarily that the soft tissue procedure needs to be more aggressive. The bony correction will be based on a false premise.

Option E is incorrect because while the CORA method is for bony correction, calculating it incorrectly based on an unadjusted radiograph will lead to an inaccurate bony correction, even if a separate soft tissue procedure is planned. The two components are interdependent in planning.

Correct Answer: C

The case explicitly addresses the 'Overcorrection Fallacy' and states, 'This approach is fundamentally flawed and should be abandoned.' It provides three main reasons: 'It Does Not Restore Stability: The lateral thrust and the underlying ligamentous instability persist. The knee remains kinematically abnormal, and shear forces continue to destroy the cartilage. It Creates an Iatrogenic Deformity: The patient is left with a visibly valgus-appearing knee, which is cosmetically unappealing and structurally unsound. It Causes Secondary Joint Pathology: Overcorrecting the tibia into valgus creates a severely oblique joint line relative to the ground. This forces the ankle into compensatory valgus, leading to secondary foot and ankle pain, and potentially subtalar joint degeneration.'

Option A is incorrect because overcorrecting the tibia into valgus would shift the load to the lateral compartment, not the medial, potentially accelerating lateral compartment pathology, not medial.

Option B is incorrect because the text clearly states, 'It Does Not Restore Stability: The lateral thrust and the underlying ligamentous instability persist. The knee remains kinematically abnormal...'

Option D is incorrect because while it might seem simpler to some, the text emphasizes its fundamental flaws and the severe consequences, making it an undesirable approach despite perceived simplicity.

Option E is incorrect because the text discusses this approach in the context of 'a varus knee with LCL laxity,' implying its use for chronic laxity associated with deformity, and still deems it flawed.

Correct Answer: C

The case discusses acute fibular head advancement and explicitly lists its limitations and risks: 'While conceptually simple, acute advancement carries significant risks and inherent limitations: High Risk to the Peroneal Nerve: The procedure requires extensive, meticulous dissection around the fibular neck, placing the common peroneal nerve at extremely high risk for traction injury (neurapraxia), entrapment, or outright transection.'

Option A is incorrect because fibular head advancement is discussed in the context of LCL laxity, which is typically associated with varus knees and lateral thrust.

Option B is incorrect because the text states, 'Limited Correction Potential: The amount of advancement is strictly restricted by the compliance of the surrounding neurovascular and muscular tissues. You can only pull the fibular head so far before tension on the nerve becomes critical.' It also mentions 'Stress Relaxation: The viscoelastic nature of ligamentous tissue means that acutely tightened ligaments tend to stretch out and relax over time, potentially leading to recurrent laxity and failure of the reconstruction,' implying that precise, durable overtensioning is difficult.

Option D is incorrect because acute correction techniques typically do not involve prolonged external fixation; that is a characteristic of gradual correction methods.

Option E is incorrect because the primary concern with acute advancement is nerve injury and limited correction, not premature fusion. Bone healing is generally desired, but the method's drawbacks outweigh this.

Correct Answer: B

The case highlights the 'profound, game-changing advantages' of the gradual fibular transport method. Specifically, it states: 'Greatly Enhanced Safety: The fibular osteotomy is made in the diaphysis, well distal to the nerve, and the transport is done gradually (typically 1 mm per day). This requires minimal dissection around the nerve itself, dramatically reducing the risk of iatrogenic nerve palsy.' And 'Simultaneous Bony Correction: The true beauty of this method is that the exact same external fixator used for the fibular transport can be utilized to simultaneously and perfectly correct any associated multiplanar tibial or femoral bony deformities.'

Option A is incorrect because the method involves a fibular osteotomy and can simultaneously correct tibial or femoral bony deformities, meaning bony osteotomies are part of the process.

Option C is incorrect because gradual correction involves a period of distraction and external fixation, which is not a faster method for immediate full weight-bearing. It's a controlled, slower process.

Option D is incorrect because it involves a fibular osteotomy and transport, which is a bone-based procedure to retension ligaments, not solely soft tissue plication.

Option E is incorrect because the method is presented as a solution for 'complex knee malalignment complicated by ligamentous incompetence,' implying its use for deformities with associated bony malalignment, not just isolated LCL tears.

Correct Answer: C

The case provides specific 'Surgical Pearl: Fibular Wire Placement' details: 'A single 1.8-mm olive wire is typically the workhorse for capturing and transporting the proximal fibula. It must be drilled from posterolateral to anteromedial directly through the fibular head, ensuring the 'olive' (stopper) is firmly seated against the posterolateral cortex of the fibula to allow for a distal pull. CRITICAL: The trajectory of this wire is paramount. It must be placed to avoid the common peroneal nerve, which runs posterior and distal to the wire's intended path. Use a wire guide and spread the soft tissues carefully down to the bone before drilling.'

Option A is incorrect because the specified trajectory is posterolateral to anteromedial, and the olive should be posterolateral.

Option B is incorrect because the wire is placed through the fibular head to transport the proximal segment, not the distal diaphysis.

Option D is incorrect because the text specifies 'A single 1.8-mm olive wire is typically the workhorse,' not multiple wires.

Option E is incorrect because the text explicitly states, 'The trajectory of this wire is paramount,' emphasizing its critical importance for both effectiveness and safety.

Correct Answer: C

This question requires integrating multiple pieces of information from the case. The patient has a varus knee deformity (MPTA 80°, normal is 87°), LCL laxity (positive lateral thrust, JLCA 5° > 2°, 4mm asymmetric lateral joint space opening on stress views). The mLDFA is normal (87°).

The Paley blueprint emphasizes a two-pronged attack: 'The preoperative plan must explicitly separate the pathology into two distinct problems that require two distinct solutions: 1. The Bony Deformity: This is corrected with a precisely calculated osteotomy (femoral, tibial, or both) to normalize the mLDFA and MPTA. 2. The Soft Tissue Deformity: This is corrected with a targeted ligamentous procedure to retension the lateral structures and normalize the JLCA.'

Given the MPTA of 80° (normal 87°), a proximal tibial osteotomy is indicated to correct the bony deformity. Given the LCL laxity (lateral thrust, JLCA > 2°, asymmetric joint space opening), a ligamentous procedure is also required. The case strongly advocates for gradual fibular transport for LCL retensioning due to its safety and effectiveness, especially when combined with bony correction using the same fixator.

Option A is incorrect because the case explicitly condemns the 'overcorrection fallacy' of intentionally creating valgus to compensate for LCL laxity, stating it is 'fundamentally flawed and should be abandoned.'

Option B is incorrect because the mLDFA is normal (87°), so a distal femoral osteotomy is not indicated. Also, acute fibular head advancement has significant risks and limitations compared to gradual transport, and the amount of tightening should correspond to the joint space difference (4mm in this case), but the method itself is less preferred.

Option D is incorrect because while the mLDFA is normal, the MPTA is 80°, indicating a significant tibial varus deformity that requires bony correction. An isolated LCL repair would not address the underlying bony malalignment.

Option E is incorrect because the mLDFA is already normal (87°), so a distal femoral osteotomy is not indicated. The MPTA of 80° is abnormal and indicates a tibial deformity, not a normal value for a varus knee.

Correct Answer: C

The case details the 'Step 2: The Fibular Osteotomy' for gradual fibular transport: 'A low-energy osteotomy is performed in the proximal fibular diaphysis. This location is chosen specifically because it is well distal to the LCL and biceps femoris insertions, and it keeps the osteotome safely away from the common peroneal nerve as it wraps around the fibular neck.' It also advises: 'Avoid using a high-speed oscillating saw, which can cause thermal necrosis and impair bone healing (regenerate formation). Instead, utilize a multiple drill-hole technique along the planned osteotomy line... to ensure a complete, clean break of the periosteum and cortex without displacing the fragments.'

Option A is incorrect because this technique is for gradual transport, not acute advancement, and the location is chosen for safety and healing, not to maximize acute movement.

Option B is incorrect because the CORA method applies to the primary deformed bone (femur or tibia) for mechanical axis correction, not typically for the fibular osteotomy itself in this context. The fibular osteotomy is for ligamentous retensioning.

Option D is incorrect because while bone healing is desired, the emphasis on low-energy technique is to prevent thermal necrosis and impairment of bone healing (regenerate formation), not necessarily to facilitate rapid consolidation beyond normal physiological rates.

Option E is incorrect because fibular transport is specifically for LCL retensioning and lateral stability, not MCL laxity.

Correct Answer: A

The case explicitly lists 'The Ability to Overtension' as a key advantage of gradual correction: 'The surgeon can intentionally overtension the LCL during the transport phase to counteract the inevitable biological stress relaxation that occurs once the frame is removed, ensuring a durable, permanent long-term result.'

Option B is incorrect because the case strongly condemns creating a compensatory valgus deformity (the 'overcorrection fallacy'). Overtensioning the LCL is about restoring ligamentous stability, not creating a new bony deformity.

Option C is incorrect because overtensioning the LCL is related to ligamentous stability, not directly to accelerating bone healing at the fibular osteotomy site. Bone healing (regenerate formation) is a separate biological process.

Option D is incorrect because the safety to the common peroneal nerve is primarily achieved by the location of the fibular osteotomy (distal to the nerve) and the gradual nature of the transport, not by overtensioning the ligament.

Option E is incorrect because gradual transport inherently involves a period of external fixation. Overtensioning is for long-term durability, not for immediate frame removal, as the bone still needs to consolidate.

Paley's Osteotomy Rule 2 dictates that if the osteotomy is at a different level than the CORA, the mechanical axes will only align if the bone ends translate relative to each other.

The normal JLCA is 0 to 2 degrees. A JLCA greater than 2 degrees that diverges laterally on a standing weight-bearing radiograph strongly indicates lateral soft-tissue laxity, primarily involving the LCL.

Normal mLDFA is 87° (range 85-90°) and normal MPTA is 87° (range 85-90°). An MPTA of 80° indicates a proximal tibial varus deformity, while the femoral alignment (mLDFA) is normal.

Correcting a combined femoral (mLDFA 94°) and tibial (MPTA 81°) varus deformity solely in the tibia creates an abnormal shear angle across the knee. This results in unacceptable joint line obliquity and accelerated articular wear.

An opening-wedge HTO proximal to the tibial tubercle lengthens the tibia distal to the joint line but does not alter the patellar tendon insertion, effectively lowering the patella relative to the joint line and causing patella baja.

Decreasing the posterior tibial slope reduces the anterior translation of the tibia relative to the femur during weight-bearing. This altered biomechanics is protective for an ACL-deficient knee.

To completely eliminate the dynamic varus thrust and restore alignment, the correction angle must account for both the bony structural deformity (MPTA) and the soft tissue laxity (abnormal JLCA component > 2°).

A lateral trunk lean toward the affected side during the stance phase shifts the body's center of mass closer to the knee joint center. This decreases the external knee adduction moment, mitigating the varus thrust.

The Fujisawa point is located approximately 62% of the way across the tibial plateau from medial to lateral. Passing the mechanical axis through this point creates 3 to 5 degrees of mechanical valgus, unloading the medial compartment.

Lateral closing-wedge HTO requires dissection near the proximal fibula and lateral tibial flare, placing the common peroneal nerve at a significantly higher risk of direct injury or tethering.

The JLCA is the angle formed between a line drawn tangent to the distal femoral articular condyles and a line drawn tangent to the proximal tibial articular plateau.

Paley's Rule 3 states that if the osteotomy and axis of hinge are outside the CORA and correction is made by angulation alone, a secondary translation deformity arises, resulting in parallel but non-collinear mechanical axes.

Normal mLDFA is 87° (range 85-90°). An mLDFA > 90° indicates a distal femoral varus deformity. Because the MPTA is normal (88°), the deformity is localized entirely to the distal femur.

In the presence of significant LCL laxity and varus thrust, overcorrecting the mechanical axis laterally ensures the knee adduction moment is neutralized during stance, dynamically tensioning the medial side and preventing lateral joint opening.

The normal JLCA ranges from 0 to 2 degrees. An elevated JLCA in a varus knee typically represents dynamic lateral joint opening due to lateral collateral ligament (LCL) laxity or significant cartilage loss.

Paley's Osteotomy Rule 1 states that if the osteotomy and the hinge (axis of correction) pass through the CORA, correction by angulation alone will realign the mechanical axes perfectly without translation. This prevents the creation of a secondary deformity.

Paley's Osteotomy Rule 2 indicates that if an osteotomy is performed at a level different from the CORA and corrected by angulation alone, the proximal and distal mechanical axes will be parallel but translated. To make them collinear, translation at the osteotomy site must accompany the angulation (Rule 3).

The normal mLDFA is approximately 88 degrees, and the normal MPTA is approximately 87 degrees. An mLDFA of 88 degrees indicates no significant femoral deformity, whereas an MPTA of 75 degrees clearly indicates a severe proximal tibial varus deformity.

The normal JLCA is 0 to 2 degrees. In a varus knee with LCL laxity, weight-bearing forces open the lateral joint space, resulting in a larger JLCA on weight-bearing or varus stress views compared to supine views. This signifies a soft-tissue contribution to the varus.

Because the proximal tibia is triangular in cross-section, opening the anterior cortex the same amount as the posterior cortex will inadvertently increase the posterior tibial slope. To maintain the native slope, the anterior opening should typically be about half the size of the posteromedial opening.

Correcting a severe deformity at a single level can result in abnormal joint line obliquity (MPTA > 95 degrees), which introduces pathological shear stresses on the articular cartilage. A double-level osteotomy is indicated to correct the mechanical axis while keeping the joint line parallel to the ground.

The mechanical axis of the lower extremity is defined by a line drawn from the center of the femoral head to the center of the tibial plafond (ankle joint). Mechanical axis deviation (MAD) is measured as the perpendicular distance from this line to the center of the knee.

A medial opening wedge HTO performed proximal to the tibial tubercle effectively elongates the proximal tibia, which relatively distalizes the tibial tubercle and decreases patellar height (patella baja). Lateral closing wedge HTO is associated with patella alta and a higher risk of peroneal nerve injury.

In the presence of lateral collateral ligament (LCL) laxity and a varus thrust, overcorrecting the mechanical axis into 3 to 5 degrees of mechanical valgus forces the knee into a valgus position during stance. This tensions the lateral soft-tissue sleeve and dynamically eliminates the varus thrust.

The parameters indicate a purely femoral deformity: the mLDFA is abnormal (94 degrees, normal ~88), while the MPTA is normal (87 degrees) and the JLCA is normal (1 degree). Therefore, a distal femoral osteotomy (such as lateral closing wedge or medial opening wedge) is required.

Paley's Osteotomy Rule 3 states that if the osteotomy is performed at a level independent of the CORA, correction requires both angulation and translation to make the proximal and distal mechanical axes collinear. This avoids the secondary translational deformity seen in Rule 2.

Creating a mechanical valgus alignment shifts the weight-bearing forces laterally. During the stance phase of gait, this alignment dynamically tensions the lateral structures (including the stretched LCL) and eliminates the lateral varus thrust, making LCL reconstruction unnecessary in most cases.

The mechanical axis of the femur (center of femoral head to center of knee) and the anatomical axis of the femur (down the medullary canal) typically diverge by about 5 to 7 degrees, with the anatomical axis being in valgus relative to the mechanical axis.

A dome (cylindrical) osteotomy allows for angular correction without significantly altering leg length. It is highly versatile, permitting adjustments in the center of rotation, though it is technically demanding.

The joint line convergence angle (JLCA) quantifies the intra-articular contribution to deformity (due to asymmetric cartilage loss or ligamentous laxity). The mLDFA and MPTA evaluate extra-articular bony deformities.

A valgus-producing HTO shifts the weight-bearing axis into the lateral compartment. Advanced lateral compartment osteoarthritis (full-thickness chondral loss) is an absolute contraindication, as the increased lateral loading will cause rapid symptom progression.

A medial opening wedge HTO elongates the proximal tibia. If the osteotomy is proximal to the tibial tubercle, it relative distalizes the patella, creating patella baja. This alters patellofemoral kinematics, often leading to anterior knee pain and extensor lag.

Because the mLDFA is normal (88 deg) and the MPTA is abnormal (80 deg), the deformity is entirely in the proximal tibia. Drawing the mechanical axis of the normal distal femur and the mechanical axis of the deformed tibia will result in an intersection (CORA) in the proximal tibia.

MAD measures the distance from the mechanical axis of the lower extremity to the center of the knee. A line passing medial to the knee center indicates varus, whereas a line passing lateral to the knee center indicates valgus.

The common peroneal nerve is highly vulnerable during a lateral closing wedge HTO, particularly during the required fibular osteotomy. It courses directly over the fibular neck, typically 2 to 3 cm distal to the tip of the fibular head, and must be carefully protected.

A supine AP or stress radiograph removes weight-bearing forces, allowing the lateral joint space to close if the increased JLCA is due to LCL laxity. If the JLCA remains abnormally wide supine, a fixed intra-articular bony deformity (like medial plateau depression) is present.

Paley's Rule 2 states that if the osteotomy is at a different level than the CORA but the hinge axis is on the CORA, the mechanical axes will realign collinearly, but the bone ends at the osteotomy site will translate.

Both the mPTA (normal 87 degrees) and mLDFA (normal 87 degrees) are significantly abnormal, indicating combined femoral and tibial deformity. Correcting this magnitude solely in one bone would result in unacceptable joint line obliquity, necessitating a double level osteotomy.

Failing to account for LCL laxity leads to undercorrection of the true bony deformity or persistent dynamic instability. Despite static neutral alignment, the unaddressed lateral laxity will allow a dynamic varus thrust during weight-bearing.

The Fujisawa point is located at approximately 62% to 62.5% of the mediolateral width of the tibial plateau, measured from the medial edge. This provides 3 to 5 degrees of mechanical valgus, effectively offloading the medial compartment.

Correcting a purely femoral deformity (abnormal mLDFA) with a tibial osteotomy will result in an excessively oblique joint line (medial down/lateral up). This creates detrimental shear forces across the articular cartilage and alters knee kinematics.

The fibular head is located posterolaterally. If left intact during a large medial opening wedge HTO, it tethers the posterolateral tibia, causing the anterior gap to open more than the posterior gap, unintentionally increasing the posterior tibial slope.

The 6-degree difference between standing and supine JLCA represents reducible lateral soft tissue laxity. Because this laxity closes when the mechanical axis is realigned, the surgeon must subtract these 6 degrees from the total standing angular deformity to prevent valgus overcorrection.

Paley's Rule 1 dictates that if the osteotomy and the hinge are both placed at the CORA, the mechanical axes will realign with pure angulation and no translational shift of the bone ends.

A varus thrust is characterized by dynamic lateral opening of the joint during stance. This is primarily permitted and exacerbated by incompetence or chronic stretching of the lateral collateral ligament (LCL) and posterolateral corner structures.

In the setting of combined varus malalignment and lateral-sided instability, the varus mechanical axis must be corrected (via HTO) to prevent excessive tensile forces from stretching out and failing a subsequent or concurrent LCL/PLC soft tissue reconstruction.

A supratubercle MOWHTO elevates the proximal joint surface while leaving the tibial tubercle distal, effectively shortening the distance between the joint line and the tubercle. This functionally lowers the patella, creating patella infera (baja) and increasing retropatellar contact pressures.