Lower Extremity Deformity Correction: Paley's Principles & Surgical Planning | Part 1

Correct Answer: B

Rationale: The CORA is the foundational concept in Paley's methodology and is defined as the point where the proximal and distal axes of a deformed bone intersect. The other options describe incorrect anatomical or conceptual points.

Correct Answer: D

Rationale: In a pure translation deformity, the proximal and distal axes are parallel and, by definition, never intersect. Therefore, the CORA is considered to be at infinity. This is a key biomechanical principle distinguishing it from angular deformities.

Correct Answer: C

Rationale: This scenario describes Paley's Rule 1. When the osteotomy and the hinge are both located at the CORA, a pure angular correction realigns the bone completely without inducing any secondary translation. This is the ideal but not always clinically achievable scenario.

Correct Answer: B

Rationale: This is a direct application of Paley's Rule 2. When the osteotomy is performed at a level different from the CORA, both angulation and translation are required at the osteotomy site to achieve a pure angular correction at the true CORA and restore the mechanical axis.

Correct Answer: D

Rationale: Paley's Rule 3 states that if the hinge of correction is placed away from the CORA, the correction will induce an iatrogenic translation deformity, which will offset the mechanical axis. This is a common technical error if the hinge placement is not precise.

Correct Answer: C

Rationale: The presence of both angulation and translation on both orthogonal views is characteristic of an oblique plane deformity (Variant 2). In this type, the deformity exists in a single plane that is oblique to the standard AP and lateral radiographic beams.

Correct Answer: C

Rationale: The text explicitly states that the definitive radiographic proof of a Variant 2 oblique plane deformity is that the CORA is located at the exact same horizontal (proximal-distal) level on both orthogonal views. This confirms that the apex of angulation and the point of translation are in the same plane.

Correct Answer: D

Rationale: The true magnitude of a deformity in an oblique plane is calculated using the Pythagorean theorem: True Magnitude = √(AP component² + Lateral component²). In this case, √(30² + 40²) = √(900 + 1600) = √2500 = 50°.

Correct Answer: C

Rationale: The graphic method is used to visualize the relationship between deformity components. When the vectors for angulation and translation lie on the same line, it confirms they share the same plane of action, which is the definition of a Variant 2 oblique plane deformity.

Correct Answer: B

Rationale: This clinical picture perfectly describes a Type 2, Variant 1 deformity. One view (AP) shows pure angulation, while the orthogonal view (lateral) shows pure translation. This indicates the plane of angulation and the plane of translation are separated by exactly 90 degrees.

Correct Answer: E

Rationale: Unlike an oblique plane deformity where the CORA is shifted, in a Type 2, Variant 1 deformity (orthogonal planes), the CORA is located exactly at the level of the malunion. This is because one view shows pure angulation, defining the apex at the fracture site.

Correct Answer: C

Rationale: The graphic method for a Type 2, Variant 1 deformity will show one vector entirely on the horizontal (mediolateral) axis and the other entirely on the vertical (anteroposterior) axis. This creates a 90-degree separation, visually confirming the orthogonal nature of the deformity.

Correct Answer: C

Rationale: The text specifically highlights the use of Poller (blocking) screws when using an intramedullary nail to correct a translated malunion. They act as a fulcrum to direct the nail and, more importantly, block the bone from shifting back into the translated deformity as the nail is passed.

Correct Answer: D

Rationale: For severe or complex deformities, especially where acute correction poses a risk to the soft tissue envelope or neurovascular structures, gradual correction with a hexapod external fixator is considered the gold standard. This allows for slow, controlled realignment, minimizing risk.

Correct Answer: C

Rationale: The text emphasizes that hexapod systems use a virtual hinge. The surgeon inputs the deformity parameters, and the software calculates a strut adjustment schedule that simultaneously corrects angulation and translation, effectively performing a perfect, gradual Paley Rule 2 correction in 3D space without needing a physical hinge.

Correct Answer: B

Rationale: The text identifies iatrogenic joint malorientation as the most common complication. By ignoring the translation and correcting the axis at the wrong level, the surgeon straightens the bone but tilts the joint line (altering the mLDFA), leading to abnormal joint mechanics and future arthritis.

Correct Answer: D

Rationale: The "Do's and Don'ts" table explicitly states to "DO obtain full-length, weight-bearing AP and LAT radiographs" and "DON'T rely on short, localized X-rays." This is because the mechanical axis can only be assessed by seeing the hip, knee, and ankle centers on a single image.

Correct Answer: D

Rationale: The surgical pearls table recommends a low-energy, multi-drill hole osteotomy (corticotomy) to preserve the periosteal blood supply. This is contrasted with a high-speed saw, which can cause thermal necrosis and inhibit healing, a major concern in deformity correction.

Correct Answer: D

Rationale: The text states that pure translation shifts the mechanical axis parallel to itself without altering joint orientation angles. Correcting only the translation at the bone level without considering the overall axis will simply move the entire malaligned axis back to the center, but the axis itself remains parallel to its deformed state.

Correct Answer: C

Rationale: The text clearly explains this pitfall: "If a surgeon only corrects the angulation and ignores the translation, the joint orientation angles may be restored, but the limb will remain mechanically malaligned (a persistent MAD)." The goal is to restore both joint orientation and mechanical axis.

Correct Answer: D

Rationale: The text explains that an X-ray beam perpendicular to the oblique plane reveals the true, maximum deformity. The apparent deformities on the AP and lateral views are merely projections (or "shadows") of this true deformity and will always be of lesser magnitude.

Correct Answer: B

Rationale: This is the conceptual opposite of the previous question. The text states, "a radiograph taken perfectly in-line with the oblique plane will show a perfectly straight bone with zero deformity." This is because you are looking "down the barrel" of the deformity plane, making it invisible.

Correct Answer: C

Rationale: The primary objective of lower limb deformity correction is to restore the Mechanical Axis Deviation (MAD) to normal, ensuring the ground reaction force vector passes through the center of the knee. This optimizes load distribution. Restoring the anatomic axis (A) is the goal of IM nailing but not the primary biomechanical goal for the limb. The other options are incorrect geometric goals.

Correct Answer: C

Rationale: The mechanical axis of a bone is defined as a line connecting the center points of the proximal and distal joints. For the femur, this is from the center of the femoral head to the center of the knee. The line bisecting the medullary canal (A) is the anatomic axis.

Correct Answer: B

Rationale: The anatomic axis is a line that bisects the medullary canal (the mid-diaphyseal line). This is particularly relevant for intramedullary fixation. The line from the center of the knee to the center of the ankle (C) describes the mechanical axis of the tibia.

Correct Answer: D

Rationale: In the femur, the anatomic axis (mid-diaphyseal line) is not parallel to the mechanical axis (hip center to knee center). The angle between them, the Anatomic-Mechanical Angle (AMA), is approximately 7 degrees. In the tibia, the axes are essentially parallel and superimposed (A).

Correct Answer: C

Rationale: The average normal mLDFA is 87° (Range 85°–90°). This angle is formed by the mechanical axis of the femur and the distal femoral joint line. An mLDFA of 81° (A) is the average for the anatomic LDFA (aLDFA).

Correct Answer: C

Rationale: The average normal MPTA is 87° (Range 85°–90°). It is a critical reference angle for assessing tibial deformity. An angle less than 85° indicates a varus deformity of the proximal tibia.

Correct Answer: D

Rationale: The average normal LPFA is 90° (Range 85°–95°). This angle is formed by the proximal femoral mechanical axis and a line from the tip of the greater trochanter to the center of the femoral head. It is used to assess for proximal femoral (hip) joint malorientation.

Correct Answer: D

Rationale: The average normal LDTA is 89° (Range 86°–92°). This angle assesses the orientation of the ankle joint relative to the tibial axis. A value greater than 92° would indicate a valgus deformity at the ankle.

Correct Answer: A

Rationale: The average normal aLDFA is 81° (Range 79°–83°). This is different from the mLDFA (87°) because it is measured relative to the anatomic axis, which is angled 7° valgus relative to the mechanical axis in the diaphysis.

Correct Answer: C

Rationale: A normal JLCA is 0°–2°. A JLCA greater than 2° is a red flag indicating that the knee joint itself is opening up, which can be due to severe cartilage wear (intra-articular deformity) or ligamentous stretching (e.g., LCL laxity in varus). It complicates planning because the tibia and femur cannot be planned in isolation.

Correct Answer: C

Rationale: Anatomic axis planning is highly practical for tibial deformities treated with an intramedullary nail because the nail will dictate the alignment of the medullary canal (the anatomic axis). This method allows the surgeon to accurately predict the postoperative alignment based on the hardware. Mechanical axis planning (B) is also valid but less direct when an IM nail is used.

Correct Answer: B

Rationale: The first step in anatomic axis planning for a diaphyseal deformity is to define the anatomic axes of the bone segments involved. This is done by drawing lines that bisect the medullary canal in the proximal and distal segments relative to the deformity.

Correct Answer: B

Rationale: If the MPTA is abnormal, it indicates a second, juxta-articular deformity. The existing proximal mid-diaphyseal line cannot be used as the reference axis. A new, "correct" anatomic axis must be drawn based on the joint line, using either the contralateral normal MPTA or the population average of 87° as the target.

Correct Answer: B

Rationale: The intersection of the proximal and distal axis lines (whether mechanical or anatomic) defines the Center of Rotation of Angulation (CORA). This is the apex of the deformity. While the osteotomy is often performed at the CORA (A), the point itself is defined as the CORA.

Correct Answer: B

Rationale: The provided text states that anatomic axis planning can be less accurate because its starting points are sensitive to tibial rotation. It explicitly mentions that if severe rotational deformity exists, mechanical axis planning is preferred. While CT-based planning (C) is an excellent modern tool, based on the provided text, mechanical axis planning is the preferred 2D radiographic method.

Correct Answer: B

Rationale: A key principle highlighted is that femoral mechanical axis planning starts *distally* at the knee by establishing the Distal Mechanical Axis (DMA). In contrast, tibial planning typically starts proximally at the knee. Starting at the hip (A) is the second step of femoral planning.

Correct Answer: A

Rationale: This scenario describes Paley's Osteotomy Rule 1. When the osteotomy and the hinge are both placed at the CORA, pure angulation occurs, and the mechanical axes of the proximal and distal segments become perfectly collinear. Rule 2 involves the osteotomy being at a different level than the CORA, which causes angulation and translation at the bone ends.

Correct Answer: B

Rationale: This is a clinical application of Paley's Osteotomy Rule 2, the basis for the angulation-translation (a-t) osteotomy. When the osteotomy is performed at a level different from the CORA (but the hinge remains at the CORA), the bone ends must be translated relative to each other to make the mechanical axes collinear. Failure to translate results in a secondary deformity.

Correct Answer: C

Rationale: Paley's Osteotomy Rule 3 states that if both the osteotomy and the hinge are placed at a level different from the CORA, a secondary translation deformity will be induced. This results in parallel but non-collinear mechanical axes and a persistent mechanical axis deviation (MAD), as described in the vignette.

Correct Answer: C

Rationale: A key reason to perform an a-t osteotomy (moving the cut away from the CORA) is to improve bone contact. By translating the fragments, the cortical corner of one segment can be impacted into the medullary canal of the other, creating a "dovetail" effect that significantly increases intrinsic stability and the surface area for healing.

Correct Answer: D

Rationale: In a valgus (knock-knee) deformity, the lateral side is convex. An opening wedge on the lateral side will place maximum stretch on the structures on this convex side. The common peroneal nerve is located laterally and is tethered at the fibular neck, making it highly susceptible to traction injury during acute valgus correction.

Correct Answer: C

Rationale: As the angular correction occurs, the convex side of the bone travels along the longest arc. This results in the greatest excursion and stretch on the soft tissue envelope on that side. Nerves, with their limited elasticity, are particularly at risk when located on the convex side of an acute correction.

Correct Answer: C

Rationale: An opening wedge osteotomy inherently lengthens the bone. For a tibia vara (bowleg) deformity, a medial opening wedge corrects the varus alignment while simultaneously increasing the length of the limb, making it the ideal choice to address both the angular deformity and the LLD.

Correct Answer: B

Rationale: The biomechanically superior location for a plate in an opening wedge osteotomy is the concave side. This placement utilizes the tension band principle. The intact cortical hinge on the convex side absorbs compressive forces, while the plate on the concave side resists tensile forces, creating a stable, load-sharing construct.

Correct Answer: C

Rationale: Placing a plate on the convex side makes it a load-bearing device. Without a structural support (like an intact hinge or bone graft) on the concave side to absorb compression, the plate is subjected to massive bending forces with every step. This cyclic loading leads to metal fatigue and eventual plate breakage.

Correct Answer: C

Rationale: Gradual correction with an external fixator is the safest and most accurate method for large, complex deformities, especially with compromised soft tissues. It avoids the acute stretch on neurovascular structures and allows for postoperative adjustments based on weight-bearing radiographs to achieve a perfect mechanical axis. The principle of distraction osteogenesis allows the soft tissues to adapt gradually.

Correct Answer: C

Rationale: The friction from a saw blade generates significant heat, which can cause thermal necrosis of the bone ends and lead to non-union. The most effective way to prevent this is to use a sharp blade (which cuts with less friction) and to dissipate heat continuously with copious chilled saline irrigation directly on the cutting surface.

Correct Answer: D

Rationale: In FAP and FAN techniques, the temporary external fixator is a reduction tool. After the osteotomy, it is used to "dial in" the perfect angular and translational correction and hold it rigidly. Once the mechanical axis is perfectly restored, the definitive internal fixation (plate or nail) is applied, and the fixator is then removed.

Correct Answer: D

Rationale: Despite its name, the dome osteotomy in long bones is not a true spherical dome. It is a cylindrical (or barrel vault) cut. This geometry allows for rotation around the central axis of the cylinder, enabling large angular corrections while maintaining excellent bone-to-bone contact.

Correct Answer: B

Rationale: The geometric principle of the focal dome osteotomy is that it behaves like a Rule 2 correction. To ensure the mechanical axes become collinear without a secondary shift, the pivot point of the correction—which is the central axis of the cylindrical cut—must be perfectly aligned with the preoperatively determined CORA.

Correct Answer: D

Rationale: Correcting a recurvatum (hyperextension) deformity involves acutely lengthening the anterior aspect of the limb. This places significant tension on the muscles and fascia of the anterior compartment, dramatically increasing intra-compartmental pressure. A prophylactic anterior compartment fasciotomy is required to prevent acute compartment syndrome.

Correct Answer: C

Rationale: The cylindrical shape of the dome osteotomy allows the bone ends to rotate along the arch, maintaining a massive surface area of contact throughout the correction. This provides excellent intrinsic stability against shear forces and promotes rapid healing, unlike a simple wedge osteotomy which creates a large gap.

Correct Answer: E

Rationale: For large bones like the femur or tibia, creating a custom, large-radius dome is technically demanding. The standard and most precise method is the multiple drill hole technique. A central K-wire acts as a pivot, and a guide is used to make a series of parallel drill holes in a perfect arc, which are then connected to complete the osteotomy.

Correct Answer: C

Rationale: The primary limitation of a standard cylindrical dome osteotomy is its inability to correct axial rotation. The bone ends are constrained by the walls of the cylinder, and any attempt to twist them results in incongruence and loss of contact. Advanced modifications like the inclined or helical dome are required for combined angular and rotational corrections.

Correct Answer: D

Rationale: The Nishio helical dome osteotomy is specifically designed to address combined angular and rotational deformities. By creating a helical or spiral cut, the osteotomy surfaces can slide along each other, allowing for simultaneous, coupled correction of angulation and axial rotation while maintaining excellent bone contact.

Correct Answer: B

Rationale: Poller, or blocking, screws are used to solve this exact problem. By placing a screw on the concave side of the deformity (in this case, medially), the surgeon artificially narrows the medullary canal. This forces the nail to follow the desired corrected mechanical axis and prevents it from shifting back into the original deformed position.

Correct Answer: C

Rationale: The "Step Plate" concept is designed to protect the screws from shear stress. The bony step abuts the plate directly, creating a path of least resistance for axial load to be transferred from the bone into the plate itself. This offloads the screws, reducing the risk of toggling, back-out, or breakage.

Correct Answer: B

Rationale: The normal PDFA is approximately 83°. A PDFA of 104° indicates a 21° distal femoral recurvatum deformity (104°-83° ≈ 20°). The PPTA of 80° is within the normal range. Since the clinical HE of 20° matches the bony femoral deformity, the source is isolated distal femoral recurvatum. Tibial recurvatum would be indicated by an abnormally high PPTA.

Correct Answer: B

Rationale: The deformity is located in the distal femur, as indicated by the abnormal PDFA of 106° (normal ~83°), representing approximately 23° of recurvatum. The PPTA is normal. The principle of deformity correction is to perform the osteotomy at the level of the Center of Rotation of Angulation (CORA), which is in the distal femur. A distal femoral flexion osteotomy will correct the alignment. A tibial osteotomy would create a compensatory deformity.

Correct Answer: D

Rationale: Correcting a femoral deformity with a tibial osteotomy creates a new, compensatory deformity. While the overall limb alignment and knee extension may become neutral, the joint orientation will be abnormal. The combination of femoral recurvatum and iatrogenic tibial flexion will make the tibia appear translated anteriorly relative to the femur, altering joint kinematics. Patella baja is a risk of tibial osteotomies proximal to the tubercle, not the primary outcome of this specific mismatch.

Correct Answer: C

Rationale: The patient has a 21° femoral recurvatum deformity (PDFA 104°). The absence of clinical hyperextension (HE=0°) indicates that the bony deformity is perfectly balanced by an equal and opposite soft tissue knee flexion contracture of 21°. This is a common compensatory mechanism. If there were no flexion contracture, the patient would exhibit 21° of clinical HE.

Correct Answer: B

Rationale: This scenario is described as the "worst of all possible treatment choices." Correcting the bony deformity without addressing the co-existing soft tissue flexion contracture will "unmask" the contracture. The osteotomy removes the bony block to flexion (the recurvatum), and the knee's maximum extension will now be limited by the soft tissues, resulting in a 20° fixed flexion deformity.

Correct Answer: B

Rationale: The total bony deformity is a 21° femoral recurvatum (PDFA 104°). The clinical presentation is only 10° of HE. The relationship is: Clinical HE = Bony Recurvatum - Flexion Contracture. Therefore, 10° = 21° - Flexion Contracture. The flexion contracture must be approximately 11° (closest answer is 10°). This represents a partially compensated deformity.

Correct Answer: C

Rationale: The ideal treatment is a compromise that corrects only the amount of the clinical hyperextension. A 10° distal femoral flexion osteotomy will reduce the 20° bony recurvatum to 10°. This remaining 10° of bony recurvatum will be perfectly balanced by the pre-existing 10° soft tissue flexion contracture, resulting in a knee that extends to 0°. This avoids creating a new flexion deformity while eliminating the clinical HE.

Correct Answer: C

Rationale: Fully correcting the 21° bony deformity will unmask the underlying 5° soft tissue flexion contracture. The knee's maximum extension will now be limited by this contracture, resulting in a 5° fixed flexion deformity. The ideal compromise would have been a 16° osteotomy to match the clinical HE.

Correct Answer: B

Rationale: In recurvatum, the knee starts in a hyperextended position. A flexion osteotomy of the distal femur effectively "pre-flexes" the distal articular segment. If the total articular arc is, for example, 120°, starting from -16° (hyperextension) would limit flexion to 104°. By performing a 16° flexion osteotomy, the starting position becomes 0°, allowing the knee to potentially reach the full 120° of flexion, thereby increasing the functional range of motion.

Correct Answer: B

Rationale: The normal PPTA is approximately 80°. A PPTA of 100° indicates a 20° proximal tibial recurvatum deformity (100°-80° = 20°). The PDFA of 84° is within the normal range. Since the clinical HE of 20° matches the bony tibial deformity, the source is isolated proximal tibial recurvatum. Femoral recurvatum would be indicated by an abnormally high PDFA.

Correct Answer: B

Rationale: The deformity is located in the proximal tibia, as indicated by the abnormal PPTA of 112° (normal ~80°), representing approximately 32° of recurvatum. The clinical HE is 24°, suggesting a small concomitant flexion contracture. However, the CORA is in the proximal tibia. The fundamental principle is to correct the deformity at its source. Therefore, a proximal tibial flexion osteotomy is the ideal treatment.

Correct Answer: C

Rationale: Correcting a tibial deformity with a femoral osteotomy creates an abnormal joint orientation. The combination of iatrogenic femoral flexion and native tibial recurvatum results in an altered patellar tracking mechanism and increased contact pressures on the patella. Patients with this combination frequently complain of patellofemoral pain and instability. The knee may also appear subluxed.

Correct Answer: B

Rationale: A PPTA of 100° indicates a 20° proximal tibial recurvatum deformity. The fact that the knee only extends to 0° (and not into hyperextension) means there must be an equal and opposite 20° soft tissue flexion contracture that is masking the bony deformity. This is a fully compensated deformity.

Correct Answer: D

Rationale: The text distinguishes between intra-articular and extra-articular causes of limited range of motion. Muscle contracture and heterotopic ossification are listed as extra-articular causes. Capsular contracture, adhesions, and articular deformity are intra-articular causes.

Correct Answer: B

Rationale: As stated in the text and shown in Figure 19-5a, the CORA for a developmental varus deformity of the hip is at the center of the femoral head. The CORA for a valgus deformity is at the base of the greater trochanter.

Correct Answer: D

Rationale: The text and Figure 19-5b indicate that for a developmental valgus deformity, the CORA is located at the lateral end of the physis of the greater trochanter, which corresponds to the base of the greater trochanter. The CORA for a varus deformity is at the center of the femoral head.

Correct Answer: B

Rationale: As described in the text and illustrated in Figure 19-7, a valgus osteotomy lengthens the femur because it makes the femoral neck more vertical. Conversely, a varus osteotomy shortens the femur by making the neck more horizontal.

Correct Answer: A

Rationale: A varus osteotomy shortens the femur because it makes the femoral neck more horizontal, decreasing the overall vertical height of the proximal femur. This is illustrated in Figure 19-7. A valgus osteotomy has the opposite effect, lengthening the femur.

Correct Answer: C

Rationale: The text explicitly states that for a valgus osteotomy, "The major extra-articular limitations to adduction of the proximal segment of the femur after osteotomy are the tensor fascia lata and the gluteus medius and minimus muscles." The adductors would be relaxed by this maneuver, not tightened.

Correct Answer: B

Rationale: As shown in Figure 19-1b, if a valgus osteotomy is performed in the presence of tight abductors that do not allow sufficient adduction of the proximal fragment, the limb will be held in abduction. This results in a fixed abduction contracture immediately after correction.

Correct Answer: C

Rationale: The text explains that valgus osteotomy has a lengthening effect, which can make the adductor muscles tight. As shown in Figure 19-1a, an adductor tenotomy may be required to improve postoperative hip abduction and relieve pressure on the femoral head caused by this increased tension.

Correct Answer: C

Rationale: The text describes a specific procedure for this scenario, based on the Harding approach. It involves "removal of the entire insertion of the gluteus medius and minimus in continuity with the quadriceps (vastus lateralis)." This allows the proximal femur to be adducted without weakening the abductors, which are reattached after the correction.

Correct Answer: C

Rationale: The text explicitly states, "It is also necessary to release the piriformis muscle, which can restrict internal rotation, extension, and adduction." The piriformis is a primary external rotator and can act as an abductor and extensor, thus its release is crucial in complex corrections.

Correct Answer: C

Rationale: As shown in Figure 19-3a, when a valgus osteotomy is performed for a varus deformity with a round head but intra-articular adduction limitation, full correction will result in a residual valgus deformity. A capsular release is required to allow the proximal fragment to adduct fully, bringing the limb into neutral alignment.

Correct Answer: B

Rationale: The text and Figure 19-3b explain that if the femoral head is non-spherical and blocks adduction, full angular correction cannot be obtained. Performing a 20-degree valgus osteotomy when only 10 degrees of adduction is available will result in a 10-degree valgus deformity, as the proximal fragment cannot be fully adducted into the corrected position.

Correct Answer: C

Rationale: The Joint Line Congruency Angle (JLCA) is the cornerstone of intra-articular deformity analysis. It is defined as the angle between a line drawn across the distal femoral condyles and a line drawn across the tibial plateaus. In a normal knee, these lines are parallel, resulting in a JLCA near 0 degrees.

Correct Answer: B

Rationale: In a healthy, congruent knee, the joint lines of the femur and tibia are nearly parallel. This relationship results in a normal JLCA that is typically measured between 0 and 2 degrees. An angle greater than this suggests intra-articular pathology such as cartilage loss or dysplasia.

Correct Answer: C

Rationale: An increased JLCA on a static weight-bearing film can be caused by true physical loss of cartilage and bone on one side, or by the joint "opening up" on the contralateral side due to ligament laxity. These two causes must be strictly distinguished during preoperative planning, as their management is different.

Correct Answer: C

Rationale: Standard weight-bearing films are insufficient to differentiate cartilage loss from ligamentous laxity. Varus and valgus stress radiographs, ideally under fluoroscopy, are the gold standard. They allow for measurement of the minimum cartilage space under compression and the maximum opening under tension, which reveals ligamentous competence.

Correct Answer: B

Rationale: Applying stress to compress the side of the deformity reveals the true remaining cartilage height. In a valgus knee, the deformity is on the lateral side, but the question asks about applying varus stress. Varus stress compresses the medial compartment, allowing measurement of the minimum cartilage space and true cartilage height on the medial side. Valgus stress would be used to assess the lateral compartment.

Correct Answer: C

Rationale: This is a common pitfall. The angulation from the JLCA must be incorporated into the total magnitude of angular correction. Failing to account for this intra-articular "wedge" will result in under-correction of the overall mechanical axis, even if the bone angles (mMPTA) are corrected perfectly.

Correct Answer: B

Rationale: Paley's Rule of Joint Line Substitution states that if one side of the joint is deformed but the other is normal, the normal side can be used as the reference for planning. In this case, with a deformed tibia and a normal femur, the distal femoral joint line serves as the reliable horizontal reference for planning the tibial correction.

Correct Answer: A

Rationale: This is the corollary to Paley's Rule of Joint Line Substitution. When there is an isolated femoral deformity with joint surface involvement and a normal tibia, the proximal tibial joint line should be used as the reference horizontal for planning the femoral correction. This prevents over- or under-correction.

Correct Answer: B

Rationale: The knee joint line should be treated as a single "best fit" line *only* if there is no intention to surgically alter the shape of the joint line itself (i.e., an extra-articular correction is planned). In this case, the preferred single best-fit line is often to adopt the normal femoral condylar line (Paley's Rule of Substitution).

Correct Answer: C

Rationale: If an intra-articular correction (like a plateau elevation) is planned, the articular surface must be characterized by two separate lines—one for the medial plateau and one for the lateral. This is the only way to accurately calculate the required millimeters of elevation and degrees of angular tilt for the depressed segment.

Correct Answer: C

Rationale: Blount's disease (tibia vara) is characterized by pathology of the posteromedial proximal tibial physis, leading to a severe depression and medial (varus) sloping of the medial tibial plateau. This creates an acute angle between the medial and lateral plateau joint lines.

Correct Answer: B

Rationale: Ellis-van Creveld syndrome (chondroectodermal dysplasia) leads to a distinct "step" depression in the lateral tibial plateau. A key feature is that the medial and lateral plateau lines often remain parallel to each other, but they are at different horizontal levels, creating a large step-off and severe valgus.

Correct Answer: C

Rationale: The fundamental geometric difference is key. The tibial plateaus are relatively flat, so a step-off creates immediate incongruity and point-loading. Conversely, the femoral condyles are round. A round surface can still roll and glide effectively on the flat tibia even if its height or orientation is altered, making the joint more "forgiving" of this type of deformity.

Correct Answer: B

Rationale: The classic "fishtail" appearance on an AP radiograph is caused by a growth arrest in the exact center of the distal femoral physis. This tethers the central portion while the medial and lateral peripheries continue to grow, causing both condyles to malorient and incline toward the middle.

Correct Answer: C

Rationale: Because of the geometric tolerance of the round femoral condyles, malalignment due to femoral unicondylar displacement or malorientation can frequently be realigned using an extra-articular distal femoral supracondylar osteotomy. A high-risk intra-articular procedure is often unnecessary if the joint is stable and functional.

Correct Answer: C

Rationale: In stark contrast to femoral deformities, the severe instability and incongruity caused by a tibial plateau step-off almost always mandates a true intra-articular osteotomy (plateau elevation). An extra-articular correction would not address the instability and would lead to rapid joint destruction.