ABOS Board Review: Lower Extremity Deformity, TKA, & Gait Analysis | Part 5

Correct Answer: D

Rationale: The text explains that to "ensure adequate foot clearance of the longer limb during swing," patients develop compensatory strategies. Vaulting involves rising on the toes of the shorter limb (in this case, the left limb, as the right is long) to create enough room for the longer right limb to swing through without dragging. The question states the right limb is long, but the vignette describes vaulting on the left, which is a common error in patient description vs. biomechanics. The text clearly states vaulting is on the *longer* limb's side to clear the *longer* limb. Re-reading the text: "vaulting (rising excessively on the toes of the longer limb)". The vignette is slightly confusing, but the principle is to clear the long limb. Let's assume the vignette meant the right limb is 4cm *longer*. Then vaulting would be on the left (shorter) side to clear the right. Let's re-read the text again. "vaulting (rising excessively on the toes of the longer limb)". This is a bit ambiguous. Let's re-read again. "To maintain forward progression and ensure adequate foot clearance of the longer limb during swing, patients develop a repertoire of energy-draining compensatory strategies. These include vaulting (rising excessively on the toes of the longer limb)". This is likely a typo in the source text. Vaulting is rising on the *shorter* limb to clear the *longer* limb. Let's answer based on the most logical biomechanical principle which is clearing the long limb. The question states the right limb has the LLD (implying it's different, let's assume longer). Vaulting on the left would be to clear the right. So D is the correct intent. Let's re-evaluate the text's statement. "vaulting (rising excessively on the toes of the longer limb)". This is biomechanically incorrect. Vaulting is plantarflexion of the *stance* limb (the shorter or normal limb) to allow clearance for the contralateral *swing* limb (the longer limb). I will answer based on the established biomechanical principle, which aligns with the *intent* of the text's sentence, even if the phrasing is slightly flawed. The goal is to clear the long limb. Let's re-read the vignette. "4 cm LLD of the right lower extremity". This usually means the right is shorter. "walks with a noticeable vaulting gait on the left side". This means he is rising on his toes on the longer left leg. This would be to clear the shorter right leg, which doesn't make sense. There is a contradiction. Let's assume the LLD means the right leg is 4cm *shorter*. Then he would vault on the right leg to clear the left leg. The vignette is flawed. Let's assume the vignette meant "4cm longer right leg". Then he would vault on the left leg to clear the right leg. This matches the vignette's description of vaulting on the left. So the goal is to clear the longer right limb. Correct Answer: D. Rationale: The text states that compensatory strategies like vaulting are employed to "ensure adequate foot clearance of the longer limb during swing." In this case, with a longer right limb, the patient vaults on the contralateral (left) limb during its stance phase to create vertical space for the right limb to swing through without catching the ground.

Correct Answer: B

Rationale: The text states, "The most consistent and telling alteration is a marked reduction in stance time on the shorter limb." This is a compensatory mechanism to minimize the vertical drop of the body's center of mass when weight is transferred to the shorter leg.

Correct Answer: C

Rationale: The text explains that when weight is transferred to a shorter leg, the center of mass drops into a "biomechanical valley." To prevent this inefficient motion and the muscular effort required to recover, "the patient rapidly offloads the short limb, rushing through the stance phase." This directly minimizes the vertical displacement of the center of mass.

Correct Answer: D

Rationale: The text describes the normal GRF curve as having a "double-hump" pattern. The second hump represents the push-off phase during terminal stance. The preoperative (top) graph shows a curve with a "severely attenuated, blunted profile" and a "severely diminished, weak push-off phase," which corresponds to the blunted second hump.

Correct Answer: D

Rationale: The text states that following successful lengthening, the GRF curve is "completely transformed." The postoperative (middle) graph shows a "robust, healthy double-hump pattern" with a "powerful, definitive push-off phase," indicating a normalization of gait kinetics that now matches the normal control graph at the bottom.

Correct Answer: B

Rationale: The text classifies LLD < 2.0 cm as "Minor." For this category, it states the discrepancy is "Often well-tolerated" and the recommended management is "Non-operative. Effectively managed with an external or internal shoe lift." Surgical options are reserved for more severe or highly symptomatic cases.

Correct Answer: B

Rationale: The patient's LLD falls into the "Moderate (2.0 - 5.0 cm)" category. The text lists "epiphysiodesis in growing children" as a primary treatment strategy for this level of discrepancy. A shoe lift would be insufficient for a 4.0 cm difference, and more aggressive options are not indicated.

Correct Answer: C

Rationale: This LLD falls into the "Severe (> 5.0 cm)" category. The text states that management for this level of severity involves "Complex surgical reconstruction. May require multi-level lengthening, external fixation (Taylor Spatial Frame), or combined lengthening and deformity correction." The other options are either insufficient or not the primary reconstructive choice described.

Correct Answer: C

Rationale: The text provides a "Crucial Diagnostic Pearl" stating that a surgeon must "absolutely rule out a *functional* LLD caused by fixed joint contractures." A knee flexion contracture makes a limb act short. The text emphasizes that "Addressing the soft tissue contracture is the primary, definitive treatment in these cases" and that lengthening the bone would be a "catastrophic surgical error."

Correct Answer: C

Rationale: The text explicitly defines the mechanical axis as "a straight line drawn from the center of the femoral head directly to the center of the ankle mortise." The other options describe incorrect anatomical landmarks for defining the mechanical axis.

Correct Answer: D

Rationale: The text states that in a normal limb, the mechanical axis "passes just slightly medial to the exact center of the knee joint (specifically at the Fujisawa point, which is approximately 62% of the way from the medial edge of the tibial plateau)." Passing significantly medial or lateral would represent varus or valgus malalignment, respectively.

Correct Answer: C

Rationale: The text defines the Mechanical Axis Deviation (MAD) as "The measured distance in millimeters from the center of the knee to the point where the mechanical axis actually crosses the joint line." A mechanical axis passing medial to the knee center indicates varus malalignment.

Correct Answer: B

Rationale: The table of joint orientation angles clearly states that the mLDFA (Mechanical Lateral Distal Femoral Angle) "Determines distal femoral varus/valgus." The MPTA assesses the proximal tibia, and the other angles assess different locations.

Correct Answer: C

Rationale: The text and table state that the normal range for the MPTA is 85° to 90°. It specifies that an MPTA < 85° indicates proximal tibial varus. An MPTA of 80° is below this threshold, signifying a varus deformity located in the proximal tibia.

Correct Answer: B

Rationale: The text explicitly states, "The single most critical factor dictating the survival of any lower extremity prosthesis is the restoration of a neutral mechanical axis." The other options are surgical choices but are secondary to achieving proper alignment.

Correct Answer: C

Rationale: The text emphasizes a paradigm shift: "They are a complex deformity correction patient who will receive prosthetic components merely as the final resurfacing step of their comprehensive reconstruction." This highlights the priority of correcting the underlying deformity.

Correct Answer: B

Rationale: The text states that eccentric loading from malalignment initiates a cascade of failure, starting with "Accelerated Polyethylene Wear" due to focused stress on a single compartment. Prosthetic fracture is a much later and less common event.

Correct Answer: D

Rationale: The text clearly identifies the cellular mechanism of osteolysis: "The generation of microscopic wear debris triggers a massive macrophage-mediated biologic response. This leads to aggressive, silent bone resorption (osteolysis)..."

Correct Answer: D

Rationale: This scenario exemplifies the "Aligned but Unstable Knee." The surgeon corrected the bone alignment but failed to address the chronic soft tissue imbalance (lax medial structures, tight lateral structures), leading to instability. A "balanced but malaligned" knee is the opposite problem.

Correct Answer: C

Rationale: The text warns against this exact scenario: "This dangerous scenario often occurs when a surgeon blindly 'chases' the soft tissues. By recutting bone to equalize gaps without respecting the overall mechanical axis of the lower extremity, the surgeon compromises the foundation."

Correct Answer: B

Rationale: The image in the bottom right (b) and the accompanying text describe this exact failure mode: "severe varus malalignment, leading to catastrophic polyethylene loss, medial bone collapse, and eventual failure due to chronic overload and wear."

Correct Answer: B

Rationale: The text is unequivocal in defining the surgical objective: "The ultimate goal of TKA is to reduce the MAD to zero," meaning the mechanical axis line passes through the center of the knee joint.

Correct Answer: C

Rationale: The text specifies that the foundation of deformity analysis "requires precise measurement of key parameters on full-length, weight-bearing radiographs." This is necessary to visualize the entire mechanical axis from the hip to the ankle.

Correct Answer: B

Rationale: The text and the bottom-left image (a) directly explain this scenario: "Because of the truncated lateral metaphysis from the prior surgery, the tibial baseplate was placed on a thin wafer of bone. The lateral peg penetrates the bone..."

Correct Answer: C

Rationale: The clinical image shows severe genu varum (bowlegs). This varus alignment concentrates forces on the medial compartment, leading to the predictable failure pattern of medial polyethylene wear, medial bone loss, and varus collapse, as described in the text.

Correct Answer: B

Rationale: The text describes this process as "Asymmetric Bone Loss and Subsidence," where overloaded trabecular bone collapses, compromising structural support and leading to the implant sinking or subsiding.

Correct Answer: B

Rationale: The text explains the soft-tissue changes in a varus deformity: "...a long-standing varus deformity will have a contracted medial collateral ligament (MCL) and a stretched, lax lateral collateral ligament (LCL)." Therefore, the MCL is the structure that would need to be released.

Correct Answer: C

Rationale: The text provides a precise definition: "The mechanical axis of the lower extremity is defined as a straight line drawn from the center of the femoral head to the center of the ankle joint (talar dome)."

Correct Answer: B

Rationale: The text defines MAD as "the measured distance (in millimeters) and direction (medial for varus, lateral for valgus) that this line deviates from the center of the knee." A medial deviation indicates a varus alignment.

Correct Answer: D

Rationale: The text specifically attributes prosthetic fracture to this mechanism: "...the sheer magnitude of non-physiologic cantilever stress can cause the metal baseplate or stem of the implant itself to fracture."

Correct Answer: A

Rationale: The text explains this outcome under "The Aligned but Unstable Knee," stating that failure to address soft tissue contractures can lead to the joint opening up as it flexes, "leading to profound mid-flexion instability, giving-way episodes, and functional limitation."

Correct Answer: C

Rationale: This is the classic "balanced but malaligned" knee. The text warns that this scenario subjects the components to "catastrophic eccentric loading," which inevitably leads to wear, osteolysis, subsidence, and failure.

Correct Answer: A

Rationale: The text summarizes the final step in failure: "The deadly combination of progressive bone loss, cantilever forces, and micromotion ultimately destroys the implant-bone interface, necessitating a massive revision surgery."

Correct Answer: C

Rationale: The text states that joint orientation angles "are the diagnostic keys for pinpointing the exact" location of the deformity, i.e., whether it is in the femur or the tibia.

Correct Answer: B

Rationale: The text introduces this core concept as "The Duality of a Successful TKA: Bone Alignment vs. Ligament Balance," emphasizing that surgeons must master both "precision bone alignment and dynamic ligamentous balance."

Correct Answer: C

Rationale: The text explicitly uses this term: "This leads to aggressive, silent bone resorption (osteolysis) around the implant components."

Correct Answer: C

Rationale: The text describes the benefit of the Paley method as follows: "These principles remove the guesswork, transforming a highly complex, anxiety-inducing problem into a series of logical, reproducible, and safe steps."

Correct Answer: B

Rationale: The Mikulicz Line, or the mechanical axis of the lower limb, is defined as the line drawn from the center of the femoral head to the center of the ankle joint. It represents the line of gravity during the stance phase. The anatomic axis (A, C) is the mid-diaphyseal line of the bone.

Correct Answer: B

Rationale: Mechanical Axis Deviation (MAD) is the perpendicular distance from the center of the knee joint to the mechanical axis of the lower limb. A lateral deviation indicates a valgus deformity. CORA (A) is the apex of the deformity, not the deviation at the knee.

Correct Answer: C

Rationale: The normal range for the mLDFA is 85°–90°, with an average of 87°. This angle is critical for orienting the knee joint parallel to the ground in the frontal plane. 90° (D) is the normal LPFA, not mLDFA.

Correct Answer: C

Rationale: Similar to the mLDFA, the normal range for the MPTA is 85°–90°, with an average of 87°. Restoring this angle is fundamental to correcting tibial deformities and ensuring a horizontal knee joint line.

Correct Answer: C

Rationale: The anatomic axis of the femur is in approximately 7° of valgus (range 5-9°) relative to the mechanical axis. This is due to the offset of the femoral head from the shaft. In the tibia, the axes are nearly parallel (A).

Correct Answer: D

Rationale: The normal Lateral Proximal Femoral Angle (LPFA) is 90°. This angle measures the orientation of the proximal femur relative to its mechanical axis. An angle greater than 90° indicates varus (coxa vara).

Correct Answer: B

Rationale: The CORA is the geometric apex of a deformity, defined as the intersection of the proximal and distal axis lines of a deformed bone. This is the fundamental starting point for Paley's deformity planning.

Correct Answer: B

Rationale: Paley's Rule 1 states that if the osteotomy and the hinge are both at the CORA, the bone will angulate and perfectly realign without any unwanted translation at the osteotomy site. This is the ideal scenario for a uniapical deformity.

Correct Answer: A

Rationale: This scenario describes Paley's Rule 2. When the osteotomy is performed away from the CORA but the hinge is at the CORA, the axis is corrected, but the bone ends will translate relative to each other at the osteotomy site.

Correct Answer: B

Rationale: This is an example of Paley's Rule 3. If both the osteotomy and the hinge are placed away from the CORA, the mechanical axis will not be perfectly realigned, and a secondary translation deformity (a "zigzag" effect) will be induced.

Correct Answer: D

Rationale: A multiapical deformity is characterized by more than one CORA, or bend, in a single bone. The classic rickets femur with bends at the proximal, diaphyseal, and distal levels is the archetypal example of a multiapical deformity.

Correct Answer: B

Rationale: The anatomic axis method is highly preferred for multiapical deformity correction with an intramedullary nail because the nail inherently follows the anatomic axis (the medullary canal). Planning along this axis directly translates to the surgical execution.

Correct Answer: C

Rationale: When correcting a multiapical deformity with a single osteotomy, the surgeon targets the "resolved apex" or resolved CORA. This is a virtual point representing the net effect of all the individual bends, and it is often located outside the bone itself.

Correct Answer: C

Rationale: A key trade-off of using a single-level (uniapical) osteotomy to correct a multiapical deformity is the creation of a "zigzag" or "Z-shaped" anatomic axis. While the mechanical axis is corrected, the bone itself has an unnatural, angulated appearance at the osteotomy site.

Correct Answer: C

Rationale: The "gold standard" multiapical correction addresses each bend individually, resulting in the restoration of a normal, straight anatomic axis. This provides a superior cosmetic and biomechanical result, especially when using an intramedullary nail. All other options (A, B, D, E) are advantages of the simpler, single-level osteotomy.

Correct Answer: D

Rationale: The zigzag deformity from a uniapical correction is most noticeable in the subcutaneous tibia, where it can create a visible and palpable bony bump. In the femur (A, B, C), the thick surrounding muscle envelope effectively camouflages the underlying bone contour.

Correct Answer: C

Rationale: In the FAN technique, the external fixator is used as a temporary reduction tool. It allows the surgeon to precisely and acutely correct the multiapical deformity and hold the bone segments rigidly in place while the intramedullary nail is inserted. The fixator is removed at the end of the case.

Correct Answer: C

Rationale: The pins for the external fixator must be placed strategically (e.g., anteriorly or anterolaterally) to avoid blocking the path of the guidewire, reamers, and the intramedullary nail itself. This is a crucial step to prevent intraoperative complications.

Correct Answer: C

Rationale: Stretching a radiopaque cord (like a cautery cord) from the center of the femoral head to the center of the ankle under fluoroscopy provides a real-time, intraoperative representation of the Mikulicz line. This allows for precise confirmation that the mechanical axis has been restored to pass through the desired point in the knee.

Correct Answer: C

Rationale: The reamings generated during intramedullary canal preparation are a rich source of autogenous bone graft. They should be preserved and allowed to pack into the osteotomy gaps, where they can significantly accelerate healing and promote union.

Correct Answer: D

Rationale: When correcting a multiapical deformity at a single level (away from the true CORAs), simple angulation is insufficient. The bone ends must also be translated to bring the proximal and distal mechanical axes into alignment. Failure to translate will result in residual MAD.

Correct Answer: C

Rationale: Large, acute corrections of the tibia can cause significant swelling and increase the risk of acute compartment syndrome. Prophylactic fasciotomy of the anterior compartment is a valid consideration to mitigate this risk. While the peroneal nerve (A) is at risk, prophylactic decompression is less common than fasciotomy for compartment syndrome prevention.

Correct Answer: B

Rationale: An opening wedge osteotomy inherently lengthens the bone on the side of the opening. A closing wedge osteotomy shortens the bone. A dome osteotomy is length-neutral.

Correct Answer: D

Rationale: While multiapical correction is the gold standard for anatomic restoration, its main drawback is the creation of multiple osteotomy sites. This doubles the number of potential locations for delayed union or non-union compared to a single-level osteotomy.

Correct Answer: C

Rationale: The provided text defines the Sagittal Mechanical Axis as a conceptual plumb line dropped vertically from the center of the femoral head. The Coronal Mechanical Axis is a line from the femoral head to the ankle center on an AP view.

Correct Answer: B

Rationale: The text states that in a normally aligned extremity, the Sagittal Mechanical Axis passes slightly anterior to the knee center. Any deviation from this, such as passing posterior to the knee, constitutes a Sagittal Mechanical Axis Deviation (MAD).

Correct Answer: C

Rationale: The text explicitly describes the method for finding the Center of Rotation of Angulation (CORA) as the intersection of the mid-diaphyseal lines of the proximal and distal bone segments. This is the most critical step in planning a corrective osteotomy.

Correct Answer: C

Rationale: The text states the normal range for aLDFA is 79° to 87°. A value greater than 87°, such as 92°, indicates distal femoral recurvatum. Proximal tibial recurvatum would be indicated by an abnormal PPTA.

Correct Answer: B

Rationale: The normal range for PPTA is 77° to 84°. According to the text, a PPTA greater than 84° indicates proximal tibial recurvatum. The value of 96° is significantly above this threshold, confirming an osseous deformity in the proximal tibia.

Correct Answer: C

Rationale: The text specifies the normal range for ADTA is 78° to 82°. A value less than 78°, such as 72°, indicates a procurvatum (flexion) deformity of the distal tibia.

Correct Answer: A

Rationale: The text states that a normal sagittal JLCA of 0° to 2° confirms that the joint surfaces are parallel and the deformity is extra-articular (osseous). The other ranges correspond to different joint orientation angles.

Correct Answer: C

Rationale: The text lists anterior physeal arrest of the distal femur or proximal tibia as a common cause of osseous deformity leading to genu recurvatum. The history of physeal arrest strongly points to a structural bowing of the bone itself.

Correct Answer: B

Rationale: The text identifies connective tissue disorders like Ehlers-Danlos syndrome as a frequent cause of ligamentous laxity, leading to incompetence of the posterior soft tissue restraints of the knee. Normal bone angles rule out an osseous etiology.

Correct Answer: C

Rationale: The text states that weakness or spasticity of key muscle groups, common in conditions like poliomyelitis, can lead to a dynamic recurvatum thrust. The absence of bony deformity and the presence of significant muscle weakness point to a neuromuscular etiology.

Correct Answer: B

Rationale: The text explains the "paradox of recurvatum gait" by stating that a patient with structural bony recurvatum uses active, dynamic muscle control to prevent the knee from snapping into its full, passive hyperextension, thus maintaining a functional gait.

Correct Answer: C

Rationale: The text specifies that in normal gait kinematics, the knee never fully extends. At heel strike, the normal knee is in approximately 5° of flexion. It then flexes further during the loading response.

Correct Answer: B

Rationale: The text and the associated diagram clearly state that in osseous knee recurvatum, the primary compensation occurs at the ankle. To get the foot flat on the ground when the tibia is angled backward, the ankle must go into compensatory plantar flexion.

Correct Answer: C

Rationale: The text explicitly states, "The deceptively normal gait pattern seen in compensated osseous recurvatum shatters completely when muscle weakness is introduced into the equation." The active muscle control required for stability is lost.

Correct Answer: D

Rationale: The text defines the normal alignment of the Sagittal Mechanical Axis as passing "slightly anterior to the center of the knee joint and directly through the center of the tibiotalar (ankle) joint."

Correct Answer: D

Rationale: The text emphasizes that the location of the osteotomy relative to the CORA "determines whether you achieve pure angular correction or inadvertently introduce a new, deleterious translational deformity."

Correct Answer: B

Rationale: The aLDFA of 83° is within the normal range (79-87°). The PPTA of 98° is significantly greater than the normal upper limit of 84°, indicating a proximal tibial recurvatum deformity.

Correct Answer: B

Rationale: The text states this specific alignment "is an evolutionary biomechanical design that allows the knee to lock efficiently during the stance phase of gait with minimal muscular energy expenditure."

Correct Answer: D

Rationale: The text highlights that mastering the joint orientation angles (aLDFA, PPTA, etc.) allows the surgeon to "move from a qualitative, subjective description... to a quantitative, actionable diagnosis." The MAD identifies that a problem exists, but the angles quantify it.

Correct Answer: C

Rationale: The normal mean aLDFA is 83°. The patient's aLDFA is 95°. The magnitude of the deformity is the difference: 95° - 83° = 12°. The PPTA is within the normal range (77-84°), indicating the deformity is in the femur.

Correct Answer: C

Rationale: The text identifies post-stroke hemiplegia as a common cause of neuromuscular imbalance leading to dynamic recurvatum. The spasticity and weakness are classic features of this etiology.

Correct Answer: C

Rationale: The text explains that patients with compensated osseous recurvatum use active muscle control to maintain normal gait kinematics, such as flexing the knee during loading response, despite the underlying bony deformity that would passively allow for hyperextension.

Correct Answer: D

Rationale: The text identifies severe, high-velocity multi-ligamentous knee injuries as a cause of ligamentous laxity leading to recurvatum. The normal bone angles rule out an osseous etiology, making the history of ligamentous injury the key diagnostic clue.

Correct Answer: C

Rationale: The introduction of the text explicitly contrasts coronal and sagittal deformities, stating that "sagittal deformities directly disrupt the kinematic chain required for forward propulsion," which is why they create such devastating functional deficits.

Correct Answer: C

Rationale: The text and diagram explain that compensatory plantar flexion at the ankle "allows the foot to reach the ground while keeping the center of gravity balanced over the base of support" when the tibia is angled backward due to recurvatum.

Correct Answer: B

Rationale: The Center of Rotation of Angulation (CORA) is the geometric point defined by the intersection of the proximal and distal axes of a deformed bone. The ACA is the hinge point of the surgical correction, which may or may not be at the CORA.

Correct Answer: C

Rationale: The Angulation Correction Axis (ACA) is the actual hinge point around which the bone segments are moved during surgery. In an opening wedge osteotomy with a plate, the intact contralateral cortex serves as the ACA. The CORA is a property of the deformity itself, not the surgical construct.

Correct Answer: C

Rationale: The Osteotomy Level is the physical location of the bone cut. It is one of the three key variables in deformity planning and is entirely determined by the surgeon based on biological and technical factors. The CORA is fixed by the deformity's geometry, and the ACA is determined by the chosen fixation method.

Correct Answer: B

Rationale: Paley's osteotomy rules are fundamentally based on the geometric relationship between the Center of Rotation of Angulation (CORA), the Angulation Correction Axis (ACA), and the physical Osteotomy Level. The final outcome of the correction is dictated by how these three variables align.

Correct Answer: C

Rationale: When the Angulation Correction Axis (ACA), the hinge of the surgery, does not coincide with the Center of Rotation of Angulation (CORA), the apex of the deformity, a secondary translation deformity is created. This is the fundamental concept behind Paley's Osteotomy Rule 3.

Correct Answer: D

Rationale: Although termed a "dome," the osteotomy is geometrically a cylindrical cut. This shape allows the bone ends to slide along a congruent arc, providing inherent stability and maximal bone contact during angular correction.

Correct Answer: C

Rationale: A key advantage of the dome osteotomy is that the congruent cylindrical surfaces maintain a large area of bone-to-bone contact, which promotes rapid primary bone healing. An opening wedge creates a large void that requires bone grafting and heals more slowly, while a closing wedge shortens the limb.

Correct Answer: D

Rationale: A dome osteotomy achieves angular correction without creating a gap (like an opening wedge) or resecting bone (like a closing wedge). Therefore, it preserves limb length while maintaining excellent bone apposition, avoiding the need for structural bone graft.

Correct Answer: B

Rationale: A significant disadvantage of the dome osteotomy is its incompatibility with rotational correction. The stability of the osteotomy relies on the congruent, matching cylindrical surfaces. Attempting to rotate one segment axially will cause the surfaces to lose contact, leading to instability and gapping.

Correct Answer: C

Rationale: As the radius of the dome osteotomy increases, the arc of the cut becomes flatter. When a large angular correction is performed on a flat arc, the bone ends pivot and lose significant surface area contact. Dome osteotomies are most effective with a tighter radius, which is achievable in wider metaphyseal bone.

Correct Answer: B

Rationale: Dome osteotomies are most practical in the wide metaphyseal regions of bone. The wider diameter allows for a tighter (smaller) radius of the cylindrical cut, which maximizes bone contact and stability during large angular corrections.

Correct Answer: B

Rationale: The "golden rule" of the Focal Dome Osteotomy is that the Angulation Correction Axis (ACA), which is the center of the cylindrical cut, is placed exactly on the Center of Rotation of Angulation (CORA) of the deformity. This ensures perfect realignment of the bone axes without secondary translation.

Correct Answer: B

Rationale: The image illustrates the principle of a focal dome osteotomy. The center of the circular arc (which represents the ACA) is placed precisely at the CORA. This ensures that as the bone segments slide along the arc, the proximal and distal axes become perfectly collinear.

Correct Answer: B

Rationale: Paley's Osteotomy Rule 2 applies when the ACA is on the CORA, but the osteotomy cut is at a different level. This is the exact principle of a focal dome osteotomy. It results in perfect realignment of the mechanical axis, but with translation of the bone ends at the osteotomy site.