Orthopedic Deformity Correction & Board Review Questions for ABOS Part I & AAOS OITE | Part 22018

Correct Answer: C

The case describes a varus malunion with a mechanical axis deviation (MAD) of 25 mm medial to the center of the knee. A medial shift of the mechanical axis (varus deformity) concentrates compressive forces on the medial compartment of the knee. This leads to accelerated medial compartment osteoarthritis due to focal cartilage overload and meniscal tears. Concurrently, the lateral collateral ligament (LCL) experiences chronic tensile strain as the knee attempts to resist the varus moment. The image, while generic, illustrates a full-length standing radiograph, which is essential for MAD assessment.

Option A is incorrect because a varus deformity places the MCL under compression, not tensile strain, and leads to medial, not lateral, compartment osteoarthritis.

Option B is incorrect because a varus deformity increases compressive forces on the medial compartment, not the lateral, and leads to medial meniscus tears, not lateral.

Option D is incorrect because patellofemoral instability and ACL laxity are typically associated with sagittal plane deformities (procurvatum/recurvatum) or altered tibial slope, not primarily a coronal plane varus deformity of the distal femur.

Option E is incorrect because a varus tibial deformity (which would also cause a medial MAD) often forces the subtalar joint into compensatory eversion, not inversion, and can limit dorsiflexion, not increase it.

Correct Answer: B

Panel (d) in the provided image clearly demonstrates a pure translation deformity. In this scenario, the proximal and distal axes of the bone are perfectly parallel, meaning they never intersect. Geometrically, the Center of Rotation of Angulation (CORA) for a pure translation deformity is theoretically located at infinity because there is no angulation, only a shift. Panels (c) and (f) depict pure angulation deformities where the CORA is located precisely at the anatomical apex of the deformity, where the proximal and distal axes intersect. Panel (e) is not explicitly described in the text but shows a CORA that is not at the apex, implying a combined angulation-translation deformity where the CORA is displaced from the anatomical apex.

Correct Answer: B

The normal mLDFA is 85-90° (average 87°), and the normal MPTA is 85-90° (average 87°). In this patient, the mLDFA is 87°, which is within the normal range. This indicates that the distal femoral coronal alignment is normal. However, the MPTA is 78°, which is significantly less than the normal average of 87°. A decreased MPTA indicates a varus deformity of the proximal tibia. The text explicitly states: 'An abnormal mLDFA with a normal MPTA isolates the deformity entirely to the distal femur.' Conversely, a normal mLDFA with an abnormal MPTA isolates the deformity to the proximal tibia. The significantly medial MAD further supports a varus deformity. The image serves as a reminder of the type of imaging required for such measurements.

Option A is incorrect because the mLDFA is normal, ruling out a distal femoral deformity.

Option C is incorrect because the MPTA specifically evaluates the proximal tibia, not the distal tibia (which is evaluated by mLDTA).

Option D is incorrect because while intra-articular pathology can contribute to MAD (assessed by JLCA), the MPTA is a bony angle, and its abnormality points to a bony deformity.

Option E is incorrect because the mLDFA and MPTA do not directly assess proximal femoral alignment.

Correct Answer: C

The provided radiographs perfectly illustrate a Variant 1 deformity of the Type 2 angulation-translation deformities. The text describes Variant 1 as having 'one view (e.g., AP radiograph) shows a pure angulation with zero translation... The orthogonal view (e.g., Lateral radiograph) shows a pure translation with zero angulation.' The image confirms this: the left (AP) view shows angulation with no step-off, and the right (Lateral) view shows parallel axes (no angulation) with a clear step-off (translation). For Variant 1, the 'Masterclass Surgical Pearl' states: 'The CORA is located exactly at the level of the malunion site.' This is because the plane where angulation exists has no translation to displace the intersection of the axes, so they cross precisely at the anatomical apex.

Option A is incorrect because this describes a Variant 2 deformity, where both views show angulation and translation, and the CORAs project at different levels.

Option B is incorrect because while the lateral view shows pure translation, the AP view shows pure angulation. The CORA for pure translation is at infinity, but for pure angulation, it's at the apex. This is a combined deformity, and the CORA for the angulation component is at the malunion level.

Option D is incorrect because it mischaracterizes the deformity and the CORA location.

Option E is incorrect because this describes a Variant 2 deformity, not Variant 1.

Correct Answer: C

The description of both AP and lateral radiographs showing both angulation and translation, with the CORA on the AP view at a different level than the CORA on the lateral view (one proximal, one distal to the malunion), is the definitive hallmark of a Variant 2 deformity. As described in the text, 'In a Variant 2 deformity, both the standard AP and Lateral radiographs will show both angulation and translation. However, there is a definitive radiographic hallmark that identifies this variant: The CORA on the AP radiograph is at a level distinctly different from that of the CORA on the Lateral radiograph.' This variant represents angulation and translation 90 degrees apart but rotated into an oblique plane. The image illustrates the concept of multiplanar deformities with angulation and translation components in different planes.

Option A is incorrect because Variant 1 deformities show pure angulation in one view and pure translation in the orthogonal view, with the CORA at the malunion level.

Option B is incorrect because a pure angulation deformity would only show angulation in one plane, and the CORA would be at the apex, not displaced differently in orthogonal views.

Option D is incorrect because a pure translation deformity would show parallel axes in both views, and its CORA is at infinity.

Option E is incorrect because the description points to a long bone malunion, not an intra-articular deformity.

Correct Answer: C

The text defines the mechanical axis as a line from the center of the femoral head to the center of the ankle joint. The perpendicular distance from the center of the knee joint to this mechanical axis line is the Mechanical Axis Deviation (MAD). A normal MAD is typically 8-10 mm medial to the center of the knee. A MAD of 20 mm medial signifies a significant varus deformity, quantifying the magnitude of the problem and indicating increased medial compartment loading. Calculating the MAD is described as 'the first, most critical step in any deformity analysis' as it 'quantifies the magnitude of the problem' and 'directly informs the magnitude of the surgical correction required to neutralize joint-loading forces.' The image shows a full-length radiograph, which is necessary for MAD measurement.

Option A is incorrect because JLCA measures ligamentous laxity or cartilage loss within the knee joint, not the overall mechanical axis deviation.

Option B is incorrect because MPTA is an angle that evaluates proximal tibial coronal alignment, not the linear distance of the mechanical axis from the knee center.

Option D is incorrect because CORA is the intersection of proximal and distal axes of a deformed bone, indicating the epicenter of angulation, not the overall mechanical axis deviation.

Option E is incorrect because PPTA evaluates proximal tibial sagittal alignment (tibial slope), not coronal plane mechanical axis deviation.

Correct Answer: D

The normal Posterior Proximal Tibial Angle (PPTA) is 77-84° (average 81°). A PPTA of 70° indicates a decreased posterior tibial slope (or increased anterior slope, depending on how it's measured, but the text implies a decrease from the normal 81°). The 'Surgical Pearl' for PPTA states: 'Altered tibial slope dramatically affects knee kinematics. Increasing slope increases anterior tibial translation and ACL strain.' While the question states a PPTA of 70°, which is less than normal, this implies a more vertical orientation of the proximal tibia, which would effectively increase the posterior slope relative to the mechanical axis if the distal segment is fixed. The text also mentions, 'An anterior translation of the distal tibia (procurvatum) effectively increases the posterior tibial slope, altering the tension on the anterior cruciate ligament (ACL) and changing patellofemoral tracking.' Therefore, a change in PPTA from the normal range, especially one that effectively increases the posterior slope, will lead to increased anterior tibial translation and increased strain on the ACL, contributing to instability and pain. The image is a generic full-length radiograph, which would be used to measure such angles.

Option A and B are incorrect as these are consequences of coronal plane deformities (varus/valgus), not sagittal plane tibial slope changes.

Option C is incorrect because an altered (increased) posterior tibial slope leads to increased, not decreased, anterior tibial translation and increased, not reduced, ACL strain.

Option E is incorrect as this is a consequence of coronal plane tibial deformities affecting the ankle, not primarily sagittal plane tibial slope.

Correct Answer: B

The text explicitly states under 'Step-by-Step Radiographic Analysis': 'On both the AP and lateral views, carefully draw the proximal and distal anatomical or mechanical axes of the deformed bone. Use the center of the intramedullary canal in the diaphyseal segments as your reliable guide.' This is crucial for accurate deformity analysis and osteotomy planning. The image represents the type of radiograph used for this planning.

Option A is incorrect because outer cortical margins can be irregular, especially in malunions, and may not accurately represent the true axis.

Option C is incorrect because joint lines define joint orientation angles, not the diaphyseal axis of the bone itself.

Option D is incorrect because periosteal reaction is a pathological finding and not a reliable anatomical landmark for axis determination.

Option E is incorrect because the most prominent point of the deformity is often the apex of angulation, but the axis needs to be drawn through the entire segment, not just a single point.

Correct Answer: C

The case describes a diaphyseal tibial malunion with varus angulation (coronal plane) and anterior translation (procurvatum) in the sagittal plane. The text specifically addresses the consequences of this combined deformity: 'An anterior translation of the distal tibia (procurvatum) effectively increases the posterior tibial slope, altering the tension on the anterior cruciate ligament (ACL) and changing patellofemoral tracking, often leading to anterior knee pain and patellar maltracking.' The image illustrates the concept of multiplanar deformities with angulation and translation components in different planes.

Option A and B are incorrect because these are direct consequences of the varus angulation component (coronal plane deformity), not the anterior translation (sagittal plane deformity).

Option D is incorrect because compensatory subtalar eversion and asymmetrical ankle wear are typically consequences of the varus angulation extending to the ankle, not primarily the anterior translation.

Option E is incorrect because anterior translation (procurvatum) effectively increases the posterior tibial slope, which leads to increased, not reduced, anterior tibial translation.

Correct Answer: C

The text describes Variant 2 deformities as having angulation and translation 90 degrees apart but rotated into an oblique plane. For these cases, 'obtaining specialized oblique radiographs can isolate the deformity. An oblique radiograph obtained perpendicular to the plane of maximum angulation will show pure angulation with no translation. Similarly, the orthogonal oblique radiograph of the plane of maximum translation will show pure translation with no angulation.' This allows for a clearer understanding of the individual components for precise planning. The image illustrates the vector components of such multiplanar deformities.

Option A is incorrect because this describes a Variant 1 deformity. In a Variant 2 deformity, both standard AP and lateral views will show both angulation and translation.

Option B is incorrect because while CT scans are useful, Paley's principles emphasize the geometric analysis on 2D radiographs, and specialized oblique views are specifically mentioned as a method to isolate components in Variant 2 deformities.

Option D is incorrect because JLCA evaluates intra-articular pathology, not the plane of a long bone deformity.

Option E is incorrect because MAD quantifies the overall malalignment but does not isolate the specific planes of angulation and translation or guide osteotomy placement in a multiplanar fashion; CORA analysis is needed for that.

Correct Answer: C

The text emphasizes the critical role of the CORA: 'The Center of Rotation of Angulation (CORA) is the absolute cornerstone of Paley's deformity analysis. Geometrically, it is the point where the proximal and distal mechanical (or anatomical) axis lines of a deformed bone intersect. The CORA is the true epicenter of the angulation.' For a pure angulation deformity (as shown in panels c and f of the image), the CORA lies exactly at the anatomical apex. The key principle is that 'Understanding exactly where the CORA is located is the single most important step in planning your osteotomy.' Performing the osteotomy precisely at the CORA allows for correction of the angulation without introducing an iatrogenic translation, thus achieving a 'perfect correction.' The image clearly shows the CORA at the apex for pure angulation.

Option A is incorrect because the CORA is at infinity for pure translation, not pure angulation.

Option B is incorrect because while neurovascular structures are a consideration, the geometric principle dictates osteotomy at the CORA for optimal correction, not just distal to it.

Option D is incorrect because the CORA is fundamental for angulation deformities; it is at infinity for pure translation, but its concept is still applied to understand the absence of angulation.

Option E is incorrect because performing an osteotomy proximal or distal to the CORA for a pure angulation deformity would introduce an iatrogenic translation, creating a new deformity rather than a perfect correction.

Correct Answer: B

The case explicitly states that a medial MAD (varus deformity) causes the mechanical axis to shift medially, pathologically overloading the medial compartment of the knee. This leads to medial meniscus tearing, subchondral sclerosis, and rapid-onset premature medial compartment osteoarthritis. A 25 mm medial MAD is a significant deviation, indicating severe varus malalignment.

Option A is incorrect because increased tensile stress on the LCL and lateral compartment overload are characteristic of a lateral MAD (valgus deformity), not a medial MAD.

Option C is incorrect because while an HTO might be considered for varus, the deformity is explicitly stated to be a malunited distal femoral fracture. Therefore, the primary source of the deformity is femoral, and a distal femoral osteotomy (DFO) would likely be indicated, not an HTO, without further assessment of joint orientation angles to pinpoint the exact source.

Option D is incorrect because a normal MAD is zero, with the mechanical axis passing directly through the center of the knee or slightly medial to the tibial spines. A 25 mm medial deviation is highly pathological and not a normal variant.

Option E is incorrect because while limb malalignment can affect patellofemoral mechanics, a medial MAD primarily impacts the tibiofemoral compartments, leading to medial compartment overload, rather than directly increasing the risk of patellofemoral instability, which is more commonly associated with valgus alignment or specific patellofemoral pathologies.

Correct Answer: B

The case explicitly states that the Medial Proximal Tibial Angle (MPTA) defines proximal tibial varus/valgus, and an abnormal value isolates the deformity to the tibia. Since the patient has a malunited proximal tibial fracture causing varus, the MPTA is the most critical angle to assess to confirm the deformity's origin in the proximal tibia.

Option A (mLDFA) is incorrect because it defines distal femoral valgus/varus and would isolate the deformity to the femur, not the tibia.

Option C (JLCA) is incorrect because it measures intra-articular deformity, cartilage loss, or ligamentous laxity, not the angular deformity of the bone itself.

Option D (mLDTA) is incorrect because it defines the orientation of the ankle joint and would isolate a deformity to the distal tibia, not the proximal tibia.

Option E (aPDFA) is incorrect because it is the sagittal plane equivalent of mLDFA, measuring distal femoral flexion or extension deformities, and is not relevant for a coronal plane proximal tibial varus deformity.

Correct Answer: C

The case explicitly states that 'pure translation still causes a massive Mechanical Axis Deviation.' It provides an example: 'a 15 mm medial translation of the distal tibia will shift the entire limb's mechanical axis 15 mm medially.' This creates a severe varus thrust at the knee, even if joint orientation angles are normal. Therefore, a 20 mm medial translation will certainly cause a significant MAD.

Option A is incorrect because the text states that in a pure translation deformity, 'the proximal and distal axes of the bone remain perfectly parallel to one another. Because these axis lines are perfectly parallel, they will extend infinitely without ever intersecting. Therefore, a pure translation deformity has absolutely no CORA.'

Option B is incorrect because the text warns against this: 'Attempting to correct a pure translation with an angular opening or closing wedge osteotomy will create a new, iatrogenic angular deformity, drastically worsening the patient's overall malalignment.' Correction of pure translation requires a simple transverse osteotomy and direct, parallel shifting of the bone segments.

Option D is incorrect because the text states that 'While the joint orientation angles (like the MPTA or mLDFA) may measure within normal limits, pure translation still causes a massive Mechanical Axis Deviation.'

Option E is incorrect for the same reason as Option B; an angular osteotomy is inappropriate for pure translation and will induce an iatrogenic angular deformity.

Correct Answer: C

The case explicitly states, 'When a bone is subjected to both angulation and translation... this intersection point—the CORA—will not be located at the anatomic apex (the visible fracture or malunion site). The translation component mathematically displaces the CORA either proximally or distally along the axis of the bone.' This displaced intersection is termed the angulation-translation point (a-t point).

Option A is incorrect because the presence of translation specifically displaces the CORA away from the anatomic malunion site.

Option B is incorrect because a CORA is absent only in pure translation deformities where axes are parallel. When angulation is also present, the axes will intersect, forming a CORA, albeit a displaced one.

Option D is incorrect because the CORA is the intersection of the proximal and distal axes of the deformed bone segment, not necessarily the knee joint center.

Option E is incorrect because the text emphasizes that ignoring this geometric shift (the displaced CORA) is 'a guaranteed recipe for surgical failure' in combined deformities.

Correct Answer: C

The case describes 'Variant 1: Anatomic Plane Deformity with Angulation and Translation 90° Apart.' It states, 'one radiograph will display pure angulation with zero translation, while the other radiograph will display pure translation with zero angulation.' The example given is 'Frontal Angulation / Sagittal Translation: The AP radiograph shows an angular deformity (e.g., varus) but the bone ends are perfectly aligned translationally. The Lateral radiograph shows no angulation (normal PPTA), but the distal segment is translated posteriorly.' This perfectly matches the clinical scenario described.

Option A is incorrect because there is clear angulation on the AP view, making it not a pure translation deformity.

Option B is incorrect because an oblique plane deformity would show both angulation and translation on both the AP and lateral radiographs, and the CORA would be at the same level on both views. Here, angulation is only on AP, and translation is only on lateral.

Option D is incorrect because this is a single deformity, just oriented in a specific way, not necessarily a multi-level deformity.

Option E is incorrect because the case provides a clear classification for this type of presentation using standard orthogonal radiographs.

Correct Answer: C

The case describes 'Variant 2: The Oblique Plane Deformity' and states, 'Consequently, both angulation and translation are projected onto both the AP and Lateral radiographs.' Crucially, it adds, 'The CORA (the a-t point) will be located at the exact same horizontal level on both the AP and Lateral radiographs.' The provided image (ch_91_fig_6c199e.webp) is explicitly used in the text to demonstrate this: 'The radiographic series above perfectly demonstrates the diagnostic process for an oblique plane angulation-translation deformity. Notice how the proximal (red) and distal (blue) axis lines intersect. The resulting CORA (marked by the black circle) is clearly displaced away from the anatomic malunion site. Crucially, when comparing the AP and lateral projections, the CORA is located at the exact same vertical distance from the joint line, confirming a single oblique plane deformity.'

Option A is incorrect because both angulation and translation are evident, and the CORA is clearly displaced from the visible malunion site, as indicated by the image and text.

Option B is incorrect because angulation is present, and the axes intersect, meaning a CORA exists.

Option D is incorrect because the alignment of the CORA at the same horizontal level on both views is the hallmark of a single oblique plane deformity, not a multi-level one.

Option E is incorrect because the presence of both angulation and translation on both views, and the displaced CORA, indicates a complex oblique plane deformity, not a simple planar one, and a simple closing wedge osteotomy would not address the translation or the oblique nature.

Correct Answer: B

The case explicitly states, 'A defining, non-negotiable feature of a single oblique plane deformity is that these two vectors [angulation and translation] will be perfectly collinear—they will lie on the exact same line, although they may point in the same or opposite directions depending on the specific geometry of the malunion.' This collinearity is crucial because it means the entire 3D deformity can be addressed by orienting the osteotomy and corrective hinge in that single oblique plane, converting a complex 3D problem into a manageable 2D correction.

Option A is incorrect because the presence of an angulation vector clearly indicates an angular component.

Option C is incorrect because collinear vectors indicate a single oblique plane deformity, not a multi-level one.

Option D is incorrect because collinearity means the vectors are in the same plane, not 90° apart. Variant 1 deformities have angulation and translation 90° apart, which would result in non-collinear vectors on the graph.

Option E is incorrect because the true magnitude is calculated using the Pythagorean theorem (hypotenuse of the triangle formed by the coordinates), not a simple sum of the apparent measurements.

Correct Answer: C

Paley's Osteotomy Rule 1 is clearly stated in the text: 'If the osteotomy is performed exactly at the CORA, and the ACA (the hinge) is also placed exactly at the CORA, the result is pure angular correction with no induced translation.' In this scenario, the CORA is at the malunion site, so performing both the osteotomy and placing the ACA at this point will achieve the desired pure angular correction.

Option A is incorrect because placing the ACA away from the CORA (even if the osteotomy is at the CORA) would induce translation, violating Rule 1.

Option B is incorrect because performing the osteotomy away from the CORA, even if the ACA is at the CORA, would fall under Rule 2, which induces translation at the osteotomy site.

Option D is incorrect because using blocking screws to induce translation is a technique for Rule 2, where translation is desired to correct a pre-existing deformity, not for pure angular correction without induced translation.

Option E is incorrect because gradual distraction with an external fixator can correct both, but the question specifically asks for pure angular correction without induced translation, which is achieved by matching the osteotomy and ACA to the CORA.

Correct Answer: B

The case describes Paley's Osteotomy Rule 2 as 'The Workhorse for Combined Deformities.' It states: 'If the osteotomy is performed away from the CORA (e.g., at the anatomic malunion site for better healing potential), but the ACA (the hinge) is placed exactly at the CORA, the correction will produce both angulation and translation at the osteotomy site.' This induced slide at the osteotomy site is mathematically perfect to neutralize the pre-existing translation. Therefore, performing the osteotomy at the malunion site and placing the hinge (ACA) at the distantly located CORA is the correct approach.

Option A is incorrect because placing both the osteotomy and ACA at the malunion site (which is away from the true CORA) would fall under Rule 3, inducing new iatrogenic translation.

Option C is incorrect because performing the osteotomy at the CORA and the ACA away from it would also induce iatrogenic translation (Rule 3).

Option D is incorrect because Rule 2 allows for simultaneous correction of both components with a single osteotomy, making two separate osteotomies unnecessary and potentially more complex.

Option E is incorrect because ignoring the CORA in a combined angulation-translation deformity is a 'guaranteed recipe for surgical failure,' as the translation component would remain uncorrected.

Correct Answer: B

The case describes Paley's Osteotomy Rule 3: 'If both the osteotomy and the ACA are placed away from the CORA, the correction will induce a new, iatrogenic translation.' While often a pitfall, the text states, 'However, in highly complex limb salvage scenarios... Rule 3 can be weaponized intentionally. The master surgeon can calculate the exact amount of iatrogenic translation that will be created by the mismatched hinge, and then plan a secondary, compensatory translational shift at the osteotomy site (using a multi-axis fixator) to achieve perfect final alignment.' This highlights the need for precise calculation and compensation.

Option A is incorrect because the text explicitly states that Rule 3 can be 'weaponized intentionally' by a master surgeon to achieve correction, implying it's not always uncorrectable.

Option C is incorrect because the 'Surgical Pearls' section emphasizes 'Soft Tissue Management,' stating, 'Correcting severe translation can place massive tension on neurovascular bundles... Prophylactic nerve releases or gradual correction via external fixation may be mandatory.' Ignoring soft tissue compromise is a major pitfall.

Option D is incorrect because a simple closing wedge osteotomy without considering the CORA or the induced translation would lead to uncorrected translation and potentially worsened mechanical axis deviation, as per the principles of combined deformities.

Option E is incorrect because hexapod circular external fixators (like the Taylor Spatial Frame) are specifically mentioned as offering 'unparalleled six-axis control' for complex oblique plane deformities. This control is precisely what would be needed to manage the calculated iatrogenic translation and perform compensatory shifts in a Rule 3 scenario, making them highly indicated, not contraindicated.

This describes Paley's Osteotomy Rule 2. When the osteotomy is outside the CORA but the hinge axis is at the CORA, the mechanical axes will realign, but the bone ends at the osteotomy site will translate.

Normal mLDFA is approximately 88 degrees (range 85-90). An mLDFA of 78 degrees indicates a significant valgus deformity originating in the distal femur. The MPTA and JLCA are within normal limits.

Ankle equinus is the most common contracture during tibial lengthening. It occurs due to the relative resistance and tension of the strong gastrocnemius-soleus complex compared to the anterior compartment musculature.

The normal proximal tibia has a posterior slope and a triangular cross-section. Due to this shape, a medial opening wedge must be slightly larger posteriorly to avoid inadvertently increasing the posterior slope.

The ideal rate of distraction is approximately 1 mm per day, typically divided into four 0.25 mm increments. A rate of 0.25 mm per day total is too slow and strongly predisposes the regenerate to premature consolidation.

A pure translational deformity has parallel proximal and distal axes that never intersect. Therefore, the CORA for a pure translation is theoretically located at infinity.

Acute correction of a severe proximal tibial valgus deformity elongates the lateral structures. This places the common peroneal nerve at high risk for a stretch-induced neuropraxia or palsy.

Fibular hemimelia is classically associated with an anteromedial tibial bow, an equinovalgus foot with absent lateral rays, and a deficient or absent fibula.

Paley's Rule 3 states that if both the hinge axis and the osteotomy are located off the CORA, the angulation will be corrected, but a new iatrogenic translation deformity will be introduced.

The normal femoral mechanical axis is a line from the center of the femoral head to the center of the ankle joint. The anatomic axis (intramedullary canal) typically diverges from this mechanical axis by about 7 degrees of valgus.

The Taylor Spatial Frame (TSF) requires accurate input of mounting parameters, deformity parameters (including CORA), and frame parameters. This enables the software to correct multi-planar and multi-apical deformities simultaneously via a virtual hinge.

A lateral MAD indicates an overall valgus alignment. The mLDFA is normal (~88 degrees), while the MPTA is abnormally high (normal ~87 degrees). An MPTA of 98 degrees signifies a severe proximal tibial valgus deformity.

The latency phase typically lasts 5 to 7 days post-osteotomy. It allows for initial hematoma formation, resolution of acute inflammation, and soft callus formation before the mechanical distraction process begins.

The transverse bisector line of the angle formed by the intersection of the proximal and distal axes contains all the geometric CORAs for that specific deformity. Placing the hinge along this line ensures pure angular correction.

Opening wedge osteotomies add a wedge of bone (or graft), generally increasing the length of the bone segment. Therefore, a lateral opening wedge will increase the overall length of the femur and the limb.

Tensioned fine wires provide a "trampoline effect," offering high bending and torsional stiffness while permitting axial micromotion. This specific mechanical environment stimulates robust bone regenerate formation in distraction osteogenesis.

For genu valgum (knock knees), growth must be tethered on the medial side to allow the lateral side to catch up. Therefore, the tension band plate is placed over the medial physis of the distal femur or proximal tibia.

Congenital pseudarthrosis of the tibia (CPT) is strongly associated with Neurofibromatosis type 1 (NF1). It typically presents with anterolateral bowing of the tibia that spontaneously fractures and fails to heal.

Normal mPPTA is typically around 81 degrees, representing normal posterior slope. In a recurvatum deformity (apex posterior), the angle between the mechanical axis and the joint line in the sagittal plane increases, often exceeding 90 degrees.

The normal JLCA is 0 to 2 degrees. An increased angle opening laterally indicates an intra-articular source of alignment change, such as lateral joint space narrowing (cartilage loss) or medial collateral ligament tightness resulting in lateral opening.

According to Osteotomy Rule 1, placing both the osteotomy and the angulation correction axis (ACA) exactly at the CORA results in collinear realignment of the mechanical axes without any translational displacement.

The classic Ilizarov method dictates a distraction rate of 1.0 mm per day, divided into a high-frequency rhythm of 0.25 mm four times daily to optimize bone regeneration and minimize soft tissue complications.

Mounting parameters in hexapod fixator software define the position of the reference ring in relation to the center of the reference bone segment (the origin). They consist of AP, lateral, and axial view offsets.

Axial stiffness of a circular frame is increased by decreasing the distance between the rings and the bone (smaller ring size), increasing wire tension, using thicker wires, and maximizing the wire crossing angle closer to 90 degrees.

To correct a valgus and procurvatum deformity in the distal fragment with a retrograde nail, blocking screws should be placed on the concave side of the deformity. Placing them lateral prevents valgus, and posterior prevents procurvatum.

Deep peroneal nerve palsy can occur during tibial lengthening due to stretch. The most appropriate initial step is to halt the distraction and acutely shorten the limb/frame by a few millimeters to relieve tension on the nerve.

Both the mLDFA and MPTA are within normal limits (85-90 degrees), indicating no extra-articular bony deformity. The abnormal varus alignment is due to an increased JLCA (normal 0-2 degrees), representing intra-articular deformity such as cartilage loss or lateral ligament laxity.

According to Osteotomy Rule 2, if the osteotomy is located outside the CORA but the ACA remains at the CORA, the mechanical axes will become collinear (angulation corrected), but the bone ends will translate at the osteotomy site.

The deformity is localized to the distal femur (mLDFA 81 degrees indicates valgus; normal is 88 degrees). To correct a valgus deformity, the medial physis must be tethered (tension band plate on the medial distal femur) to allow the lateral side to catch up.

Paley's Osteotomy Rule 1 states that when both the osteotomy and the axis of rotation (hinge) are located at the Center of Rotation of Angulation (CORA), pure angular correction is achieved without any translation of the bone fragments.

Because the proximal tibia is triangular in cross-section (narrower anteriorly), opening the gap equally at the anterior and posterior margins will inadvertently increase the posterior tibial slope. To maintain the native slope, the anterior opening should typically be roughly half the size of the posterior opening.

During tibial lengthening, the intact soft tissues will pull the distal fibula proximally if it is not secured to the tibia. Proximal migration of the lateral malleolus leads to a loss of lateral talar support and subsequent ankle valgus deformity.

Paley's Rule 1 states that when the osteotomy and the ACA both pass through the CORA, the deformity corrects by pure angulation without any translation. This maintains the collinearity of the proximal and distal anatomical/mechanical axes.

Paley's Rule 2 dictates that if the ACA is at the CORA but the osteotomy is at a different level, the axes will correctly realign, but angulation and translation will occur at the osteotomy site. The translation is necessary to achieve collinearity of the axes.

The JLCA measures the angle between the articular surfaces of the distal femur and proximal tibia. An increased JLCA (>2 degrees) typically indicates intra-articular cartilage loss, subchondral bone defects, or collateral ligament laxity.

The TSF is a hexapod external fixator based on the Stewart-Gough platform, which allows simultaneous multi-planar correction (6 degrees of freedom) using a single computer-generated strut adjustment schedule.

The normal mLDFA is 87 degrees, with an accepted physiological range of 85 to 90 degrees. Deviations outside this range indicate a coronal plane deformity in the distal femur.

The latency period allows for the initial inflammatory and soft callus phases of bone healing to begin. For metaphyseal corticotomies in distraction osteogenesis, 7 to 10 days is the standard latency period.

Ilizarov established that a rate of 1 mm per day is optimal, but divided into frequent, smaller increments (rhythm) to protect soft tissues and promote better ossification. The standard is 0.25 mm four times daily.

Guided growth is reversible, but overcorrection can occur if plates are left in too long. Additionally, a "rebound phenomenon" (return of the deformity) is common after plate removal, occasionally necessitating intentional slight overcorrection during treatment.

Lengthening over a nail allows the external fixator to be removed immediately after the distraction phase is complete, as the intramedullary nail supports the bone during the consolidation phase. This dramatically decreases the time the patient must wear the frame.

In infantile Blount disease (stages I-II) in a child under age 3-4, KAFO bracing is the initial treatment of choice. Surgery is indicated if bracing fails, the child is older than 4, or the disease is advanced (stage III+).

A medial closing wedge osteotomy is inherently stable due to broad cancellous bone contact, allowing high union rates without the need for structural bone graft. However, it does cause a slight shortening of the limb.

Paley's Rule 3 states that if both the ACA and the osteotomy are outside the CORA, the mechanical axes will remain parallel but not collinear, creating a new translational deformity (malalignment) at the end of the correction.

As the tibia is lengthened, the surrounding soft tissues, particularly the Achilles tendon and gastrosoleus complex, become relatively tight and resist stretch. This commonly leads to an equinus contracture if not aggressively managed with physical therapy, splinting, or surgical lengthening.

When the proximal and distal axes are parallel but not collinear, the intersection point (CORA) is mathematically at infinity. This defines a pure translational deformity.

The mechanical axis of the femur is a line drawn from the center of the femoral head to the center of the knee joint (intercondylar notch). This is fundamental for assessing mechanical axis deviation (MAD).

Docking site nonunion or delayed union is a common complication in bone transport. Routine management involves surgical preparation of the docking site with decortication, removal of fibrous tissue, and placement of autologous bone graft to ensure union.

Malalignment specifically describes the mechanical axis of the limb not passing through the normal center of the knee (abnormal MAD). Malorientation refers to the lines of the joints not being at their normal physiological angles relative to the bone axes (e.g., abnormal mLDFA or MPTA).

This is a minor pin-tract infection (e.g., Checketts-burns Grade 1 or 2). Given the pin is stable without radiographic loosening, the standard initial treatment is a short course of oral antibiotics targeting skin flora (e.g., Staphylococcus) and enhanced local pin care.

If the intersection of the proximal and distal axes (the apparent CORA) lies completely outside the bone contour, it implies that the true deformity is either a pure translation or it consists of more than one apex (a multi-apical deformity).