Orthopedic Surgery Board Exam Review: Lower Limb Deformity, Paley's Principles & TKA | Part 22006

Correct Answer: C

The correct answer is C, the Medial Proximal Tibial Angle (MPTA). The case describes a patient with medial Mechanical Axis Deviation (MAD) and a suspicion of proximal tibial varus. According to Paley's principles, the MPTA is the cornerstone of tibial alignment in the frontal plane. A normal MPTA ranges from 85° to 90° (average ~87°). An MPTA of less than 85° specifically indicates tibia vara, which is a proximal tibial varus deformity. High Tibial Osteotomies (HTOs) are designed to correct this angle, typically restoring it to a normal or slightly valgus alignment (e.g., 89-90°) to offload the medial compartment.

Option A (mLDFA) is the mechanical Lateral Distal Femoral Angle. While critical for overall limb alignment, it assesses distal femoral alignment (normal 85-90°). An abnormality here would indicate a femoral deformity, not a proximal tibial one.

Option B (LDTA) is the Lateral Distal Tibial Angle. This angle assesses distal tibial alignment in the frontal plane (normal 86-92°). An abnormality here would indicate a deformity at the ankle level, not the proximal tibia.

Option D (PPTA) is the Posterior Proximal Tibial Angle. This angle assesses the sagittal plane alignment (posterior tibial slope) of the proximal tibia (normal 77-84°). While important for knee kinematics, it does not directly quantify frontal plane varus/valgus deformity.

Option E (NSA) is the Neck-Shaft Angle. This angle assesses the frontal plane alignment of the proximal femur (normal 124-136°). It is unrelated to knee or tibial deformities.

Correct Answer: D

The correct answer is D, the Posterior Proximal Tibial Angle (PPTA). The case explicitly states that the posterior tibial slope, which is directly related to the PPTA (normal 77-84°, equating to ~9° posterior slope), is 'absolutely critical for knee kinematics and ligamentous stability.' An increased posterior slope (meaning a decreased PPTA) shifts the resting position of the tibia anteriorly, thereby increasing strain on the Anterior Cruciate Ligament (ACL). Conversely, a decreased slope (increased PPTA) places excessive strain on the Posterior Cruciate Ligament (PCL). Therefore, evaluating the PPTA is paramount in a patient with ACL deficiency, as an abnormal slope could contribute to instability or impact the success of reconstruction.

Option A (mLDFA) is the mechanical Lateral Distal Femoral Angle. This angle assesses frontal plane distal femoral alignment (normal 85-90°) and is not directly related to sagittal plane ACL strain.

Option B (PDFA) is the Posterior Distal Femoral Angle. This angle assesses sagittal plane distal femoral alignment (normal 79-87°). While important for overall knee extension/flexion, it does not directly dictate the tibial slope's impact on ACL strain.

Option C (MPTA) is the Medial Proximal Tibial Angle. This angle assesses frontal plane proximal tibial alignment (normal 85-90°) and is primarily relevant for varus/valgus deformities, not sagittal plane ligamentous stability.

Option E (LDTA) is the Lateral Distal Tibial Angle. This angle assesses frontal plane distal tibial alignment (normal 86-92°) and is relevant for ankle deformities, not knee ligamentous stability.

Correct Answer: C

The correct answer is C. The case explicitly states under 'Surgical Pearls for Joint Orientation Mapping' that for sagittal plane knee lines, 'The posterior distal femoral line in children is uniquely drawn at the physis, not the articular cartilage, because the cartilaginous anlage is not fully ossified and cannot be reliably seen on plain films.' This is a critical distinction for pediatric patients, as relying on the unossified articular cartilage would lead to inaccurate measurements.

Option A describes the method for drawing the frontal plane distal femoral joint orientation line in adults, not the sagittal plane line, and doesn't account for the physis in children.

Option B describes the mechanical axis of the femur, not a joint orientation line.

Option D describes the hip joint orientation line used to calculate the mLPFA, not the distal femoral sagittal plane line.

Option E describes the frontal plane proximal tibial joint orientation line, not the distal femoral sagittal plane line.

Correct Answer: C

The correct answer is C, the mechanical Lateral Distal Femoral Angle (mLDFA). The case describes a patient with lateral MAD, indicating a valgus deformity, and the suspicion is that it originates from the distal femur. The mLDFA is defined as the angle between the mechanical axis of the femur and a line tangential to the most distal points on the convexity of the two femoral condyles. A normal mLDFA ranges from 85° to 90° (average ~88°). An abnormally low mLDFA (less than 85°) indicates distal femoral valgus (knock-knees), which would cause a lateral shift of the mechanical axis. Distal femoral osteotomies (DFOs) are specifically designed to correct the mLDFA.

Option A (MPTA) assesses proximal tibial alignment (normal 85-90°). An abnormality here would indicate tibia vara or valga, not a distal femoral deformity.

Option B (LDTA) assesses distal tibial alignment (normal 86-92°). An abnormality here would indicate an ankle-level deformity, not a distal femoral one.

Option D (PDFA) assesses sagittal plane distal femoral alignment (normal 79-87°). While related to the distal femur, it describes flexion/extension deformities, not frontal plane valgus.

Option E (NSA) assesses proximal femoral alignment (normal 124-136°). It is unrelated to distal femoral or knee deformities.

Correct Answer: C

The correct answer is C, the Lateral Distal Tibial Angle (LDTA). The case explicitly states that the LDTA is formed when the frontal plane ankle joint orientation line (across the tibial plafond) intersects the mechanical axis of the tibia. A normal LDTA ranges from 86° to 92° (average ~89°). An LDTA less than 86° indicates distal tibial varus, which shifts contact pressures medially and aligns with the patient's presentation of ankle varus deformity and medial gutter impingement. The text further emphasizes that 'Correcting the LDTA is a primary goal in supramalleolar osteotomies (SMOT).'

Option A (MPTA) assesses proximal tibial alignment (normal 85-90°) and is relevant for knee deformities, not ankle varus.

Option B (mLDFA) assesses distal femoral alignment (normal 85-90°) and is relevant for knee deformities, not ankle varus.

Option D (ADTA) is the Anterior Distal Tibial Angle. This angle assesses sagittal plane ankle orientation (normal 78-82°) and is relevant for procurvatum/recurvatum deformities, not frontal plane varus.

Option E (PPTA) assesses sagittal plane proximal tibial alignment (normal 77-84°) and is relevant for knee kinematics and ligamentous stability, not frontal plane ankle varus.

Correct Answer: D

The correct answer is D. The normal Neck-Shaft Angle (NSA) ranges from 124° to 136° (average ~130°). An NSA of 115° is significantly less than the normal range. The case defines an NSA less than 120° as coxa vara. The clinical implication of coxa vara is that it 'shortens the limb and can lead to a Trendelenburg gait due to abductor weakness' because the abductor muscles lose their mechanical advantage (moment arm) due to the altered femoral neck geometry.

Option A is incorrect because coxa valga is an NSA greater than 135°, and while it affects the abductor moment arm, it typically increases it, not decreases it in the context of a Trendelenburg gait.

Option B is incorrect because coxa vara leads to a decreased abductor moment arm, resulting in weakness, not an increased one.

Option C is incorrect because coxa valga (NSA > 135°) would lead to a relatively longer limb, and the patient has coxa vara.

Option E is incorrect because an NSA of 115° is clearly outside the normal range (124-136°), indicating a significant deformity.

Correct Answer: C

The correct answer is C. Under the 'Surgical Pearls for Joint Orientation Mapping' section, the case explicitly states: 'When performing an opening or closing wedge osteotomy, the hinge pin must be perfectly parallel to the joint orientation line to prevent inducing an unwanted secondary deformity in the orthogonal plane.' This principle ensures that the correction occurs purely in the intended plane (e.g., frontal plane for varus/valgus) without inadvertently creating a deformity in the sagittal plane (e.g., flexion/extension).

Option A is incorrect. While the mechanical axis is crucial for overall alignment, hinge pins are oriented relative to the joint line to control the plane of correction, not necessarily perpendicular to the mechanical axis of the entire limb.

Option B is incorrect. Hinge pins are oriented relative to the joint line, not necessarily parallel to the anatomic axis of the bone, especially if the anatomic axis itself is deformed or if the osteotomy is near a joint.

Option D is incorrect. While the CORA is the apex of the deformity and the ideal location for an osteotomy, the hinge pins' orientation (parallel to the joint line) is a separate, critical principle for preventing secondary deformities, regardless of whether the osteotomy is precisely at the CORA or away from it (using Paley's rules).

Option E describes the goal of traditional Mechanical Alignment in TKA, not a general principle for hinge pin placement in osteotomies. Furthermore, achieving 90-degree angles for mLDFA and MPTA might not be the goal for all osteotomies, especially if aiming for kinematic alignment or specific overcorrection.

Correct Answer: B

The correct answer is B. The case clearly differentiates Mechanical Alignment (MA) and Kinematic Alignment (KA) in TKA. It states: 'Mechanical Alignment: Traditional TKA aims to cut the distal femur and proximal tibia perpendicular to their mechanical axes. This forces the mLDFA and MPTA to be exactly 90°.' In contrast, 'Kinematic Alignment: Modern KA techniques aim to restore the patient's pre-arthritic joint orientation lines. The surgeon intentionally cuts the tibia at an MPTA of 87° and the femur at an mLDFA of 87° to match the native anatomy...'

Option A incorrectly swaps the definitions. MA aims for a neutral MAD (by making mLDFA and MPTA 90°), while KA aims to restore native joint lines.

Option C is incorrect. NSA and LDTA are not the primary distinguishing factors between MA and KA; the focus is on the knee's frontal plane angles (mLDFA, MPTA).

Option D is incorrect. Both MA and KA consider both frontal and sagittal planes, but the fundamental difference lies in their approach to frontal plane joint orientation (perpendicular to mechanical axis vs. native joint line).

Option E is incorrect. While 87° and 88° are average normal values, MA specifically aims for 90° for both mLDFA and MPTA, which often alters the native joint line. KA aims to restore the individual patient's pre-arthritic angles, which might be around these averages but are not rigidly set to them for all patients.

Correct Answer: D

The correct answer is D. The case states that the normal ADTA (Anterior Distal Tibial Angle) ranges from 78° to 82° (average ~80°). An ADTA of 70° is significantly decreased from the normal range. The clinical implication section for ADTA explicitly states: 'If the ADTA is decreased (e.g., <75°), the joint is in recurvatum, leading to posterior impingement and loss of plantarflexion.' This perfectly matches the patient's symptoms of chronic posterior ankle pain and limited plantarflexion.

Option A describes a frontal plane deformity (distal tibial varus) associated with an abnormal Lateral Distal Tibial Angle (LDTA), not ADTA.

Option B describes a frontal plane deformity (distal tibial valgus) also associated with an abnormal LDTA, not ADTA.

Option C describes ankle procurvatum, which occurs when the ADTA is increased (e.g., >85°), leading to anterior impingement and loss of dorsiflexion. This is the opposite of the patient's ADTA and symptoms.

Option E is incorrect because an ADTA of 70° is clearly outside the normal range, indicating a significant deformity.

Correct Answer: B

The correct answer is B, the mechanical Lateral Proximal Femoral Angle (mLPFA). The case defines the mLPFA as the angle formed when a line from the proximal tip of the greater trochanter to the center of the femoral head intersects the mechanical axis of the femur. A normal mLPFA ranges from 85° to 95° (average ~90°). The clinical implication section for mLPFA explicitly states: 'The mLPFA is a brilliant measure of the articulotrochanteric distance (ATD). If the tip of the greater trochanter rides too high (e.g., due to a collapsed femoral neck or coxa vara), the mLPFA decreases. A high-riding trochanter creates functional abductor weakness because the gluteus medius loses its tension and resting length.' This directly explains the patient's high-riding trochanter and abductor weakness.

Option A (NSA) is the Neck-Shaft Angle. While coxa vara (a low NSA) can lead to a high-riding trochanter, the mLPFA is the direct measure of the articulotrochanteric distance and its impact on abductor mechanics, as described in the text.

Option C (mLDFA) assesses distal femoral frontal plane alignment and is unrelated to hip abductor mechanics.

Option D (MPTA) assesses proximal tibial frontal plane alignment and is unrelated to hip abductor mechanics.

Option E (PDFA) assesses distal femoral sagittal plane alignment and is unrelated to hip abductor mechanics.

Correct Answer: C

The question describes the Mechanical Axis Deviation (MAD), which is the perpendicular distance from the mechanical axis of the entire lower limb to the center of the knee joint. According to the case, the normal MAD is approximately 8 mm (± 7 mm) medial to the center of the knee. An excessive medial deviation (i.e., the mechanical axis passing significantly medial to the knee center) indicates a varus alignment, which predisposes to medial compartment osteoarthritis. Conversely, a mechanical axis passing lateral to the knee center indicates a valgus alignment.

Option A is incorrect because the normal MAD is medial, not lateral, and 0 mm is not the typical normal value.

Option B is incorrect because the normal MAD is medial, not lateral.

Option D is incorrect because the normal MAD is approximately 8 mm medial, not 0 mm, and an excessive medial deviation indicates varus, not valgus.

Option E is incorrect because 15 mm is outside the normal range, and an excessive medial deviation indicates varus, not valgus.

Correct Answer: B

Paley's 'Less Than 90 Degrees' rule states that when an axis line intersects a joint line, the angle that is normally less than 90° is chosen for formal naming and reference. For the distal femur, the intersection of the mechanical axis and the knee joint line creates the mMDFA (mechanical Medial Distal Femoral Angle) and the mLDFA (mechanical Lateral Distal Femoral Angle). The normal mLDFA is 88°, while the normal mMDFA is 92°. Therefore, the mLDFA is the standard reference angle.

Option A is incorrect because while mMDFA is 92°, it is not the angle chosen by the 'less than 90 degrees' rule.

Option C refers to the anatomic axis (aLDFA), which is 81°, but the question specifically refers to the mechanical axis intersection described by the resident's measurements (88° and 92°).

Option D is incorrect because mMDFA is normally 92°, not 88°.

Option E is incorrect because aMDFA is not the standard reference, and the value is incorrect for the anatomic axis.

Correct Answer: B

According to the case, the normal mLDFA (mechanical Lateral Distal Femoral Angle) is 88° (range: 85–90°). An mLDFA > 90° indicates femoral varus. The patient's mLDFA of 95° clearly indicates femoral varus.

The normal MPTA (Medial Proximal Tibial Angle) is 87° (range: 85–90°). An MPTA < 85° indicates tibial varus. The patient's MPTA of 80° clearly indicates tibial varus.

Therefore, both the femur and the tibia are contributing to the patient's overall varus deformity.

Option A is incorrect as both measurements indicate varus, not valgus.

Options C and D are incorrect because both bones are contributing to the varus deformity, not just one in isolation.

Option E is incorrect as both femoral and tibial alignments are abnormal and indicate varus, not valgus.

Correct Answer: C

The case explicitly states that the Posterior Proximal Tibial Angle (PPTA) represents the natural posterior slope of the tibial plateau, and its normal value is 81° (range: 77–84°). Maintaining this angle is critical during HTO to avoid altering knee kinematics and cruciate ligament tension.

Option A (PDFA) is the Posterior Distal Femoral Angle, which is normally 83°, but it relates to the distal femur, not the proximal tibia.

Option B (PPFA) is the Posterior Proximal Femoral Angle, normally 90°, related to the proximal femur.

Option D (ADTA) is the Anterior Distal Tibial Angle, normally 80°, related to the distal tibia/ankle joint.

Option E (aJER) is the anatomic axis to joint edge ratio, which describes an intersection point, not an angle, and 1/5 is the ratio for the proximal tibia, but it's not the angle itself.

Correct Answer: C

The case states that if a patient has a severe varus or valgus deformity, but the mLDFA and MPTA are normal, the deformity is intra-articular. A widened JLCA (Joint Line Convergence Angle) on the lateral side indicates lateral collateral ligament laxity or severe medial cartilage loss in a varus knee, and conversely, a widened medial JLCA indicates medial collateral ligament laxity or severe lateral cartilage loss in a valgus knee. The normal JLCA is 0–2°, meaning the distal femoral and proximal tibial joint lines should be nearly parallel. In this patient, normal mLDFA and MPTA with a severe valgus deformity and a significantly widened medial aspect of the knee joint strongly suggest an intra-articular deformity, specifically medial collateral ligament laxity, leading to an increased JLCA.

Options A and B are incorrect because the mLDFA and MPTA are stated to be within normal limits, ruling out primary femoral or tibial bony deformities.

Options D and E describe varus deformities, which contradict the patient's presentation of a valgus deformity.

Correct Answer: D

The case explicitly states, 'The ONLY time the prefix MUST be used is for the Distal Femoral Angle. Because the anatomic and mechanical axes of the femur diverge by about 7°, the mLDFA (88°) and aLDFA (81°) are vastly different, yet both are less than 90°. You must always specify mLDFA or aLDFA.'

Option A is incorrect; the mechanical and anatomic axes of the femur are NOT collinear; they diverge by about 7°.

Option B is incorrect; the normal values for mLDFA (88°) and aLDFA (81°) are different.

Option C is incorrect; the anatomic axis (aLDFA) is used, for example, during intramedullary nailing or when the full mechanical axis cannot be visualized.

Option E is incorrect; the 'less than 90 degrees' rule applies to the distal femur, but the unique aspect is that both the mechanical and anatomic angles are less than 90°, necessitating the prefix.

Correct Answer: C

The case states under 'Anatomic Axis to Joint Center Distance (aJCD)' that 'The anatomic axis intersects the proximal femur precisely at the piriformis fossa. This is why the piriformis fossa is the traditional starting point for straight antegrade femoral nails.'

Option A is incorrect; the mechanical axis connects to the center of the femoral head, but the anatomic axis intersects at the piriformis fossa.

Option B describes the intersection of the anatomic axis with the distal femur (knee joint line).

Option D describes the intersection of the anatomic axis with the proximal tibia (knee joint).

Option E describes the intersection of the anatomic axis with the distal tibia (ankle joint).

Correct Answer: C

The case details the 'Anatomic Axis to Joint Edge Ratio (aJER)' in the sagittal plane:

• 'Proximal Tibia: The anatomic axis intersects the tibial plateau at an aJER of 1/5 (i.e., very anteriorly).'
• 'Distal Tibia: The anatomic axis intersects the ankle joint exactly in the middle, an aJER of 1/2.'

Therefore, the correct combination is Proximal Tibia: 1/5 and Distal Tibia: 1/2.

Options A, B, D, and E present incorrect combinations or values for the aJER of the proximal and distal tibia.

Correct Answer: B

The case highlights 'The 7-Degree Rule': 'The difference between the mechanical and anatomic axes of the femur is typically 7° (range 5-9°). This is why a standard distal femoral cutting block in Total Knee Arthroplasty (TKA) is usually set to 5° or 7° of valgus—it is converting the anatomic axis (the intramedullary rod) into a mechanical axis cut perpendicular to the load-bearing line.'

Option A is incorrect; there is a significant difference between the two axes.

Option C is incorrect; the correction is typically in valgus, not varus, to achieve a perpendicular cut to the mechanical axis.

Option D is incorrect; the difference is 7°, not 15°.

Option E is incorrect; the difference is 7°, not 2°, and the correction is valgus, not varus.

Correct Answer: B

The case outlines the systematic malalignment test as follows:

1. Measure the MAD: Is it medial (varus) or lateral (valgus)?
2. Measure the mLDFA: Is the femur contributing to the deformity?
3. Measure the MPTA: Is the tibia contributing to the deformity?
4. Measure the JLCA: Is there ligamentous laxity or cartilage loss contributing to the deformity?

The fundamental rule of osteotomy (Paley's Rule 1) states: 'if an osteotomy is performed at the CORA, and the bone ends are angulated to correct the deformity, the mechanical axis will be perfectly restored without any translation.'

Option A is incorrect because the sequence is wrong, and the rule states 'without translation'.

Option C is incorrect because the sequence is wrong, and the rule states 'without translation'.

Option D is incorrect because the sequence is wrong, and the rule states 'without translation', not 'with rotation'.

Option E is incorrect because the sequence is wrong, and the rule states 'without translation', not 'with rotation'.

According to Paley's Osteotomy Rule 2, if the osteotomy is placed at a different level than the CORA but the hinge axis is at the CORA, the correction will result in angulation with co-linear translation. This perfectly aligns the proximal and distal mechanical axes without creating an iatrogenic deformity.

According to Paley's Rule 2, if the osteotomy is at a different level than the CORA but the hinge is placed on the CORA, the mechanical axis will be restored. However, collinear translation of the bone segments will occur at the osteotomy site.

Extra-articular coronal plane deformities >20 degrees in the femur generally require a corrective osteotomy rather than compensatory intra-articular resection. Intra-articular correction of such large extra-articular deformities disrupts collateral ligament balance and joint line obliquity.

An increased JLCA indicates intra-articular deformity or ligamentous imbalance. In a varus knee, a JLCA > 2 degrees (converging medially/open laterally) typically reflects asymmetric cartilage wear medially and stretching/laxity of the lateral soft tissues.

In a PCL-deficient knee, increasing the posterior tibial slope shifts the tibia anteriorly relative to the femur under axial load. This reduces the posterior sag and protects the PCL/PLC reconstruction.

The latent period of 5-7 days allows for early callus formation, primary angiogenesis, and cellular proliferation at the corticotomy site. This ensures robust regenerate bone formation during the subsequent distraction phase.

The Anatomic-Mechanical Angle (AMA) of the femur normally ranges from 5-7 degrees. In an accentuated lateral femoral bow, the anatomic axis diverges more from the mechanical axis, necessitating a larger valgus cut angle (e.g., 7-9 degrees) when using an intramedullary guide.

Paley's Rule 3 states that if the osteotomy and the hinge are both placed away from the CORA, the mechanical axis of the newly aligned segment will be parallel to the target axis, resulting in a residual translation deformity.

Patella baja is common after closing wedge HTO, making patellar eversion difficult and dramatically increasing the risk of patellar tendon avulsion. A tibial tubercle osteotomy (TTO) safely improves exposure and allows for proximalization of the tubercle to correct the baja.

The mLDFA (normal 85-90 degrees) evaluates the coronal plane alignment of the distal femur. An abnormal mLDFA indicates a distal femoral deformity, distinguishing it from tibial or intra-articular sources of malalignment.

A type II valgus knee is defined by medial collateral ligament attenuation. If the knee cannot be balanced with soft tissue releases due to medial incompetence, a Constrained Condylar Knee (CCK) implant is required to provide coronal stability.

The deformity is entirely in the proximal tibia (abnormal mPTA of 80; normal mLDFA of 88). Cutting the tibia perpendicular to its mechanical axis and the femur at its standard anatomic valgus angle, followed by medial release, will correct this extra-articular deformity intra-articularly without issue.

For multi-apical deformities, correcting both apices simultaneously without inducing translation requires an osteotomy and hinge at each CORA. A three-ring construct provides a stable reference segment for each CORA, allowing independent corrections.

Excessive posterior tibial slope functionally increases the flexion gap relative to the extension gap. This causes the femur to slide anteriorly (or tibia posteriorly) in deep flexion, leading to flexion instability.

In a medial closing wedge DFO, the mechanical and anatomical hinge should be placed at the lateral cortex. Leaving an intact cortical hinge laterally provides intrinsic stability, guides the closing wedge, and facilitates rapid union.

Paley's Rule 2 states that if the osteotomy is performed at a level different from the CORA, but the ACA is placed exactly at the CORA, the mechanical axes will fully realign. However, this realignment occurs at the expense of translation at the osteotomy site.

A severe anterior bowing of the femoral diaphysis causes a straight intramedullary rod to pitch anteriorly at the distal end. This results in the cutting block being directed into flexion, leading to a flexed femoral component and potential notching or limited knee extension.

A Krackow Type II valgus deformity is characterized by an incompetent/attenuated MCL. Standard unconstrained implants (CR, PS) will fail due to medial instability, requiring a higher level of constraint such as a CCK or rotating hinge prosthesis.

The JLCA measures the angle between the articular lines of the distal femur and proximal tibia. An increased JLCA typically signifies intra-articular pathology, such as asymmetric joint space narrowing (cartilage wear) or collateral ligament laxity.

Opening wedge HTOs (and closing wedge to a lesser extent) typically lower the joint line and can lead to secondary patella baja. This scarring and relative shortening of the patellar tendon make patellar eversion and adequate surgical exposure during subsequent TKA very challenging.

Because the normal anatomical axis of the femur is oriented in roughly 6-9 degrees of valgus relative to the mechanical axis, lengthening strictly along the anatomical axis without compensation shifts the mechanical axis laterally, creating a valgus alignment.

Severe valgus deformities are classically associated with lateral femoral condyle hypoplasia. Referencing the posterior condyles directly in this setting will internally rotate the femoral component, adversely affecting patellofemoral tracking and flexion gap balance.

When a multi-apical deformity is corrected with a single osteotomy, the ACA must be placed at the intersection of the proximal and distal mechanical axes. Correcting at this single point inevitably leads to translation at the osteotomy site to achieve collinear alignment.

The normal mLDFA is approximately 87 degrees (range 85-90 degrees). An mLDFA greater than 90 degrees indicates a distal femoral varus deformity, while an angle less than 85 degrees indicates valgus.

Intra-articular compensatory bone cuts during TKA can typically manage extra-articular femoral coronal deformities up to 20 degrees. Deformities >20 degrees in the femur (or >15 in the tibia) usually necessitate a staged or concurrent extra-articular osteotomy to avoid collateral ligament compromise.

The normal Posterior Proximal Tibial Angle (PPTA) is 81 degrees (range 77-84 degrees). An increased PPTA (e.g., 95 degrees) signifies a proximal tibial apex posterior deformity, which clinically manifests as genu recurvatum.

The classic sequence for medial release in a varus knee begins with removal of osteophytes, followed by release of the deep MCL and posteromedial capsule. If further release is needed, the superficial MCL is progressively elevated.

Paley's Rule 3 dictates that if both the osteotomy and the ACA are placed independently of the CORA, the correction will result in translation AND a newly introduced angulation deformity, failing to make the mechanical axes collinear.

In a lateral opening wedge distal femoral osteotomy, the hinge must be maintained at the medial cortex. Preserving this medial cortical hinge is critical to prevent displacement/translation of the distal segment and to provide inherent stability for the bone graft and fixation.

Retained hardware in the femoral canal precludes the use of standard intramedullary alignment guides. Computer navigation or robotic assistance allows the surgeon to accurately determine the mechanical axis and make precise bone cuts extramedullary.

The mLDFA is abnormal at 94 degrees (normal is ~87 degrees), indicating a distal femoral varus deformity. The mMPTA is normal (87 degrees), confirming that the proximal tibia is not the primary osseous source of the mechanical axis deviation.

The anterior crest of a bowed tibia curves anteriorly away from the mechanical axis. If the guide is placed parallel to this bowed crest, the cutting block pitches posteriorly, resulting in a cut with excessive posterior slope.

Blount's disease (tibia vara) classically presents as a complex 3D deformity consisting of varus angulation, internal tibial torsion, and procurvatum (anterior bowing/flexion deformity) of the proximal tibia.

Mounting parameters in a hexapod frame system (like the TSF) specifically define the exact spatial relationship (AP, lateral, and axial alignment) between the reference ring and the reference bone segment. This allows the software to accurately calculate the strut adjustments.

Intra-articular compensatory cuts for extra-articular deformities are limited by the anatomy of the knee. If the required bone cut is so severe that it would excise or irreparably compromise the origins or insertions of the collateral ligaments, an extra-articular osteotomy must be performed instead.

According to Paley's principles and guidelines for TKA in extra-articular deformity, if the mechanical axis of the proximal segment passes through the knee joint, an intra-articular cut can generally be safely performed. If it passes outside the knee, an extra-articular corrective osteotomy is typically required to avoid collateral ligament compromise.

Paley's Osteotomy Rule 2 states that if the osteotomy is at a different level than the CORA but the hinge axis is at the CORA, the correction will result in both angulation and translation of the bone ends. This collinear realignment successfully restores the mechanical axis.

The patient has a medial MAD (varus deformity) with a normal mLDFA (87 degrees) and normal JLCA. Therefore, the varus deformity must originate in the proximal tibia, corresponding to an abnormally low mMPTA (normal is 85-90 degrees, so 80 degrees indicates proximal tibial varus).

A high joint line convergence angle (JLCA) indicates intra-articular deformity or ligamentous laxity. In a valgus knee with a medially opening JLCA, this reflects stretching or laxity of the medial collateral ligament (MCL).

Excessive anterior femoral bowing causes the rigid IM rod to be directed more anteriorly in the distal femur. If the cut is based on this uncompensated trajectory, it leads to relative extension of the femoral component, potentially tightening the flexion gap.

The normal posterior proximal tibial angle (PPTA) is approximately 81 degrees (range 77 to 84 degrees). This corresponds to the native posterior slope of the tibial plateau, which is roughly 9 degrees relative to the perpendicular of the anatomic axis.

In a femur with excessive lateral bowing, the IM canal directs the rod in varus relative to the mechanical axis. To compensate and avoid a varus resection, the IM rod entry point should be moved medial to the true center of the anatomic notch.

The proximal tibia has a triangular cross-section. To maintain the native sagittal slope during a medial opening wedge HTO, the anterior gap must be about half the size of the posteromedial gap. Opening the anterior gap excessively will tilt the plateau posteriorly, increasing the posterior tibial slope.

Paley's Rule 3 states that placing both the osteotomy and the hinge (axis of correction) away from the CORA will result in pure angular correction at the osteotomy site but will cause translation of the mechanical axis, creating a new mechanical axis deviation (a 'zig-zag' deformity).

The patient has significant varus deformity originating from BOTH the distal femur (mLDFA > 90 degrees) and the proximal tibia (mMPTA < 85 degrees). A single-level correction would result in an unacceptable joint line obliquity, necessitating a double level osteotomy.