FIGURE 28-1 Lateral radiograph of a swollen knee in a 3-month-old girl who reportedly fell out of her crib 8 days earlier. Subperiosteal ossification along the distal femoral shaft indicates separation of the distal femoral epiphysis. Note evidence of fracture separation of the proximal tibial epiphysis as well. Final diagnosis: Abused child.

Pathologic Fractures

Underlying conditions such as neuromuscular disorders, joint contractures, or nutritional deficiencies may predispose some children, regardless of age, for separation of the distal femoral epiphysis.^3–5,51,63Like other pathologic fractures, distal femoral physeal separations that occur in children with underlying conditions typically result from low-energy mechanisms, such as inadvertent twisting of the limb during transferring from a bed or stretching during physical therapy. Nonambulatory children, such as children with cerebral palsy, are particularly susceptible to pathologic fractures due to disuse osteopenia. Ambulatory children with spina bifida may develop epiphysiolysis, or a chronic separation of the distal femoral physis, and be unaware of it because of altered sensation. Salter–Harris fractures of the distal femur have been reported during manipulation of the knee under anesthesia in children who had developed knee contractures secondary to arthrofibrosis after treatment of displaced tibial eminence fractures.⁸⁶

Biomeachanics of the Injury of Fractures of the Distal Femoral Physis

In the adolescent with open growth plates about the knee, the most common mechanism of fracture of the distal femoral physis is a varus or valgus stress (Fig. 28-2) across the knee joint from a direct blow or buckling while landing from a jump or fall from a height. In most cases, this medially or laterally directed force is coupled with a torsional moment from direct application of force to the foot, or more commonly, from twisting of the knee on the planted foot. In an animal model, the physis is least able to resist torsional forces.¹⁵ Knee hyperextension or hyperflexion forces result in sagittal plane displacement. The combination of forces applied to the physis, however, determines the direction of displacement of the distal fragment.

Loading the limb to failure across the immature knee is more likely to lead to physeal disruption due to tensile stresses that are transmitted through the ligaments to the adjacent physis than it is to disruption of the major knee ligaments.²⁵ Varus or valgus forces (Fig. 28-3A) create tension on one side of the physis and compression on the opposite side. The result is the disruption of the periosteum, which may become entrapped between the epiphysis and the metaphysis, and the perichondrial ring on the tension side, followed by a fracture plane that begins in the hypertrophic zone and proceeds in an irregular manner through the physis.¹⁵ In adults, a similar mechanism of injury is more likely to cause ligamentous disruption rather than bone failure because ligaments of the mature knee are less able to withstand extreme tensile forces compared to the bone of the adult distal femur and proximal tibia (Fig. 28-3B).

Associated Injuries with Fractures of the Distal Femoral Physis

Because many of these injuries are the result of high-energy mechanisms such as traffic accidents and motor sports, associated visceral injuries occur in approximately 5% of patients.²⁴ Other musculoskeletal injuries are seen in association with distal femoral physeal fractures in 10% to 15% of patients.^24,83 Other long bone fractures, as well as pelvic and spine fractures, must be ruled out, especially if the mechanism of injury is high-energy motor trauma (Fig. 28-4). Knee ligament disruption, however, is the most common concomitant musculoskeletal injury. Knee instability is diagnosed in 8% to 37% of patients^11,24 and is typically diagnosed after fracture healing with the initiation of rehabilitation and return to activities. Salter–Harris type III fractures of the medial femoral condyle are most frequently associated with anterior cruciate ligament injuries.^16,54,68,85 Open fractures and vascular injuries are uncommon associated injuries, occurring in about 3% of patients. Peroneal nerve injury occurs in about 2% to 7% of patients with displaced fractures.^10,24

Signs and Symptoms of Fractures of the Distal Femoral Physis

Presentation

Emergency department assessment for children who are victims of high-energy trauma with a suspected distal femoral physeal separation should be initially evaluated by the trauma team to identify potential life-threatening injuries, to evaluate the ABCs, and to initiate resuscitation protocols if indicated. On the initial survey, head trauma, thoracoabdominal injuries, unstable spine and pelvic fractures, and limb-threatening extremity injuries are the priorities. After stabilization of the cardiovascular status, a thorough secondary survey should focus on the extremities. Long-bone fractures and ligamentous injuries of the extremities are identified with a careful orthopedic examination of all four extremities. Although severe injuries may occur in association with fractures of the distal femoral physis, this fracture, however, occurs as an isolated injury in most patients.

For patients with displaced distal femoral physeal fractures, the diagnosis may be obvious. Patients typically describe severe pain, giving way of the limb and obvious knee deformity after a sports injury, motor vehicle accident, or other high-energy mechanism and are unable to walk or bear weight on the injured limb. On examination, visible limb malalignment, severe swelling, and often ecchymosis at the apex of the knee deformity are identified. In fractures with severe displacement, the skin at the apex may be tented or puckered from protrusion of the metaphyseal distal femur through the periosteum and quadriceps muscle into the dermis. Hematoma may be palpable beneath the skin. Abrasions or laceration of the overlying soft tissues may be a clue to the mechanism of injury or to an open fracture (Fig. 28-5). Assessment of knee range of motion and ligament stability is not possible in most cases with obvious displacement because of pain and the poor reliability of the examination in the face of fracture instability. Aggressive manipulation is also potentially harmful to the fractured physis or neurovascular structures that are already compromised.

Patients with nondisplaced fractures are more difficult to diagnose. Many children with nondisplaced distal femoral physeal fractures present with knee pain or mild knee swelling after a twisting injury or blow to the knee but are able to bear weight, albeit with often a painful limp. Point tenderness at the level of the distal femoral physis, either medially or laterally about the knee, is perhaps the most reliable way to detect this injury. Range of motion is typically painful but may not be severely restricted in all cases, and fracture crepitus is absent because the periosteum is not fully disrupted. Varus/valgus stress testing of the knee ligaments is usually painful and, in some cases, may reveal subtle movement or suggest instability. The examiner, however, must be mindful that a skeletally immature patient with point tenderness at the physis is more likely to have sustained a physeal fracture of the distal femur, compared to disruption of the medial or lateral collateral ligaments of the knee. Therefore, forceful or repeated stress testing of the knee in these cases should be avoided to minimize trauma to the injured physis.

Motor and Sensory Testing

Careful neurovascular examination of the lower leg and foot must be performed for all children with suspected fractures of the distal femoral physis, especially for those with obvious limb deformity. Complete motor and sensory testing of the distal limb is necessary to identify injury of the sciatic nerve and its branches, the tibial and common peroneal nerves. Because the peroneal nerve is injured in about 2% of patients with displaced fractures¹⁰ and is the most commonly injured nerve related to this fracture,²⁴ it is especially important that anterior (deep branch) and lateral (superficial branch) compartment muscle function and lower leg sensation be carefully documented. This nerve injury is typically a neurapraxia, the result of stretching from anterior or medial displacement of the distal femoral epiphysis.

Vascular Assessment

Although vascular injuries are rare after fractures of the distal femoral physis,^24,50,71,83 the vascular status must also be evaluated carefully. The distal pulses are palpated in the foot and ankle and other signs of adequate perfusion are evaluated. These other signs include assessment of capillary refill, skin temperature, and signs of venous insufficiency such as distal swelling or cyanosis. Doppler ultrasound and measurement of ankle-brachial indices are methods available in the emergency department which are useful for detecting less obvious vascular injury when pulses and other signs are equivocal. Laceration, intimal tear, and thrombosis in the popliteal artery may occur by direct injury to the artery by the distal end of the metaphysis when the epiphysis is displaced anteriorly during a hyperextension injury.^10,24,74 Because anteriorly displaced fractures have an increased risk of neurovascular damage in general compared to other directions of displacement,^21,80 the patient must be particularly suspicious for a vascular injury with obvious hyperextension deformity of the knee.

Compartment syndrome after distal femoral physeal fracture is rare but in one series occurred in 1.2% of patients.²⁴ Signs of compartment syndrome in the lower leg such as severe swelling, tenseness or tenderness of compartments, and examination abnormalities consistent with the diagnosis are also evaluated. Compartmental pressure recordings should be obtained if there are clinical findings of compartment syndrome of the lower leg. Compartment syndrome in association with this fracture is more likely to manifest hours after injury; however, not at the time of initial presentation. Patients at risk for developing a delayed compartment syndrome after fracture are those with other injuries of the lower leg, such as tibial shaft fractures, and those with compromised vascularity.²⁴

Imaging and Other Diagnostic Studies for Fractures of the Distal Femoral Physis

Radiographs

High-quality orthogonal radiographic views of the femur and knee are for diagnosing distal femoral physeal separations (Table 28-1). On the AP radiographic, physeal widening and the presence of a fracture line proximally in the metaphysis or distally in the epiphysis allows the surgeon to differentiate between the four most common Salter–Harris types, i.e., types 1 to 4. In addition, epiphyseal varus (also called apex lateral angulation) or valgus (also called apex medial angulation) and medial or lateral translation in the coronal plane are determined on the AP view. The lateral projection defines the amount of angulation and translation of the epiphysis in the sagittal plane. The anteriorly displaced epiphysis is usually tilted so that the distal articular surface faces anteriorly. This direction of displacement is alternatively called hyperextension of the epiphysis or apex posterior angulation. The posteriorly displaced epiphysis is tilted downward so that the distal articular surface faces the popliteal fossa, sometimes described as hyperflexion of the epiphysis or apex anterior angulation. Minor degrees of displacement may be difficult to measure on plain films unless the x-ray projection is precisely in line with the plane of fracture. Even small amounts of displacement are significant.^37,50 Rotational malalignment of the distal fragment relative to the proximal fragment may be identified on either view and is dramatic in some cases with severe displacement.

Diagnosis of minimally displaced distal femoral physeal fractures is challenging. Because the physis normally is radiolucent, injury is typically identified because of physeal widening, epiphyseal displacement, or metaphyseal bone injury suggestive of a fracture. Without obvious radiographic abnormalities, nondisplaced Salter–Harris type I or III fracture without separation can be easily overlooked.^5,72,85 Oblique views of the distal femur may reveal an occult fracture through the epiphysis or metaphysis. In the past, stress views of the distal femur were recommended for patients with negative radiographs who have an effusion or tenderness localized to the physis.⁷⁸ However, it is our practice to forego stress radiographs because they are painful to the patient and may damage the already compromised physis. Presumptive S-H I fractures are then either immobilized for 1 to 2 weeks and reexamined or are further evaluated with MRI.^54,78,81,85

MRI

MRI is the most commonly used advanced imaging study for evaluating traumatic knee injuries in children and adolescents. The primary utility of MRI is to identify acute knee injuries when the examination and radiographs are nondiagnostic or to confirm diagnostic suspicions. In one large MRI study of 315 adolescents with acute traumatic knee injuries, physeal injuries of the distal femur were diagnosed in seven patients with negative plain radiographs.¹⁹ MRI also facilitates identification of knee ligament tears, meniscal pathology, and osteochondral fractures that may occur concomitantly with distal femoral physeal fractures,⁵⁴ both in the acute setting and after fracture healing. MR arteriography is one method of evaluating vascular anatomy and flow in patients with an abnormal vascular examination in association with displaced distal femoral physeal fractures.

CT Scan

Computed tomography (CT) scan is recommended for all patients with Salter–Harris III and IV fractures diagnosed on plain radiographs. In one study, CT identified fracture displacement and comminution that was not recognized on plain radiographs of the knee. The authors encouraged its use for evaluation of these fractures to identify displacement, define fracture geometry, and plan surgical fixation.⁴⁶ CT may also be useful to identify fractures and displacement in cases where the plain radiographs are negative but the examination is suspicious for a distal femoral physeal fracture.

Special Situations of Fractures of the Distal Femoral Physis

Neonate

Separation of the distal femoral epiphysis in a neonate is particularly difficult to diagnose on initial X-rays unless there is displacement, because only the center of the epiphysis is ossified at birth. This ossification center is in line with the axis of the femoral shaft on both AP and lateral views in normal infants. Any degree of malalignment of the ossification center from the shaft should raise suspicion for this fracture. Comparison views of the opposite knee and other modalities may also be helpful to identify its presence in neonates when radiographs of the affected leg are equivocal. MRI, performed under anesthesia, is another commonly used diagnostic imaging study that may help to identify a separation of the unossified femoral epiphysis.⁸⁸

Unique to the neonate is the use of ultrasonography³⁵ to evaluate distal femoral physeal separation. Typically used to evaluate the immature hip for developmental dysplasia of the hip, diagnostic ultrasound imaging may also be used to evaluate the cartilaginous distal femur in a young child with incomplete ossification of the distal femoral epiphysis. Although this study is safe and readily available, it is unfamiliar to many technicians and radiologists, making its reliability questionable unless performed by an experienced team. This modality may be used not only to diagnose injuries but also to guide reduction. Knee arthrography, another option for evaluating the immature distal femoral epiphysis for possible disruption, is primarily used to facilitate reduction and fixation in the operating room.

Physeal Arrest

The best method for determining the viability of the physis after healing of a traumatic injury is MRI performed with fat-suppressed three-dimensional spoiled gradient-recalled echo sequences.²² Impending growth disturbance can be identified early with this MRI^22,26 technique and MRI can be used to map the extent of physeal bony bar formation to determine if excision is an option for treatment.^22,49 Although CT may also be used to map the location and area of physeal bars, it is out preference to use MRI because it does not expose the child to radiation and evaluates the quality of the physeal cartilage adjacent to the bar, a possible predictor of the success of physeal excision.

Classifications of Fractures of the Distal Femoral Physis

Salter–Harris Classification

Several types of classification schemes have been used to describe fractures of the distal femoral physis, each with some merit because of the information that its use provides to the surgeon. The Salter–Harris classification⁷⁴ is the most widely used classification scheme (Fig. 28-6). This familiar classification system, based on plain radiographs, is useful for the description of the types of physeal fractures of the distal femur. As opposed to its application to other physeal fractures, however, the Salter–Harris scheme is not as reliable in predicting the risk of growth disturbance as it relates to the fracture types.^24,50 For many physeal fractures in other anatomic sites, risk of growth disturbance is smaller after type I and II fractures and higher after types III and IV. Distal femoral physeal fractures, however, are at risk for significant growth disturbance regardless of type.^7,50,82 This classification scheme is useful for treatment planning and is also a good indicator of the mechanism of injury.²¹

Salter–Harris I Fractures. The Salter–Harris type I pattern is a fracture that traverses the distal femoral physis, without extension either proximally into the metaphysis or distally into the epiphysis or knee joint (Fig. 28-7). Anatomically, this fracture cleaves the physis predominately across the physeal zones of cell hypertrophy and provisional calcification. Because of the undulation of the distal femoral physis, likely evolutionarily developed to increase the stability of the physeal plate when subject to shear stress, most distal femoral physeal fractures do not propagate cleanly across these zones but instead also extend into the germinal zones of the physis. This encroachment of the fracture line into cartilage precursor cells is likely the explanation for increased rates of growth disturbance after S-H I and S-H II fracture types.

Although this fracture pattern may be seen in any age group of skeletally immature patients, it occurs more frequently in infants, the result of birth trauma or abuse, and in adolescents with sports-related trauma. Many S-H I fractures are nondisplaced and may go undetected. Sometimes, the diagnosis is made only in retrospect, after subperiosteal new bone formation occurs along the adjacent metaphysis, evident on follow-up radiographs 10 to 14 days after injury or by MRI. When displacement is present before the age of 2 years, it usually occurs in the sagittal plane. Approximately 15% of physeal fractures of the distal femur are type I fractures.⁷

Salter–Harris II Fractures. The Salter–Harris type II pattern is the most common type of separation of the distal femoral epiphysis (Fig. 28-8). This pattern is characterized by a fracture line that extends through the physis incompletely and then exits proximally via an oblique extension of the fracture line through the metaphysis. The metaphyseal corner that remains attached to the epiphysis is called the Thurston Holland fragment. Although the direction of displacement varies, typically the direction of displacement is also the location of the metaphyseal fragment because the metaphyseal spike occurs on the side of compression forces. This fracture type may also be seen in children of all ages but is more common in adolescents. Slightly more than half (57%) of all distal femoral physeal fractures are S-H II fractures.⁷

Salter–Harris III Fractures. The Salter–Harris type III injury has a fracture line that traverses part of the physis then exits distally, with extension of the fracture line vertically across the physis, epiphysis, and its articular surface (Fig. 28-9). Most Salter–Harris type III injuries of the distal femur traverse the medial physis and extend into the joint, separating the medial condyle from the lateral condyle of the distal femur. These injuries are often produced by valgus stress across the knee, the same mechanism of injury that produces medial collateral and cruciate ligament disruption in skeletally mature patients may have an associated injury to the cruciate ligaments.^16,66 This fracture occurs most frequently in older children and adolescents and comprises about 10% of all distal femoral physeal fractures.⁷

Nondisplaced S-H III and IV fractures and other more complex patterns of distal femoral physeal fracture may not always be detectable or fully delineated on plain radiographs, requiring MRI or CT to identify.^45,53,54,58 It has been hypothesized that the Salter–Harris type III fracture, seen mostly in older children and adolescents, may occur as a consequence of the progression of closure of the distal femoral physis. This pattern of fracture occurs near skeletal maturity when the central portion of the distal femoral physis begins to close before the medial and lateral parts of the physis, similar to a juvenile Tillaux fracture of the distal tibia.⁵⁴ Occasionally, a type III fracture may occur in the coronal plane of the distal femoral condyle, more commonly the medial femoral condyle, similar to the “Hoffa fracture” of the posterior condyle seen in adults.^45,58 This fracture is difficult to diagnose with standard x-rays⁷²and is also challenging to reduce and fix. A triplane fracture of the distal femur, a fracture that appears as an S-H I injury in the sagittal plane and an S-H III fracture in the coronal and sagittal planes, has also been described.⁵³ This triplane fracture is not completely analogous to the classic triplane fracture of the ankle, however, because, while the fracture line extends in three dimensions about the physis, the distal femoral physis is completely open.

Salter–Harris IV Fractures. In Salter–Harris type IV injuries of the distal femur, the fracture line extends vertically through the metaphysis, across the physis, ultimately extending through the epiphysis and its articular surface (Fig. 28-10). It is at times difficult to distinguish between S-H III and S-H IV fractures because the metaphyseal fragment may be small and difficult to identify on plain radiographs. S-H III and IV fractures likely occur from similar mechanisms and in the same age ranges, with both presenting management challenges that require anatomic realignment of the joint line and physis to minimize risk of growth disturbance. Of fractures of the distal femoral physis, this fracture type is seen slightly more frequently than type III fractures, accounting for about 12% of fractures.⁷

Salter–Harris V Fractures. When initial radiographs of the distal femur are normal but subsequent imaging months after the traumatic injury identify a growth arrest, this fracture is termed a Salter–Harris V.⁷⁹ It is hypothesized that compression forces across the physis causes damage to the cartilage-producing cells in the growth plate but no epiphyseal displacement. Axial loading of the limb, such as from a fall from a height, is considered the classic mechanism of injury. It important to bear in mind, however, that premature growth arrest also may occur in association with nonphyseal fractures of the femoral and tibial shafts.^8,33,57,76 MRI may identify bone contusion on both sides of the growth plate after a traumatic injury that may be a harbinger to its occurrence.⁷⁶Approximately 3% of physeal separations of the distal femur are Salter–Harris V fractures.

Salter–Harris VI. Rang^59,70 proposed a sixth type of Salter–Harris fracture that applies to the distal femur in children and adolescents with open growth plates. A type VI injury is an avulsion fracture of the periphery of the physis, resulting in an osteocartilaginous fragment comprising a portion of the perichondrial ring of the physis as well as small pieces of metaphyseal and epiphyseal bone. These may occur at many different anatomic sites but are seen most commonly about the physes of the distal fibula, distal femur, and distal tibia.³² The mechanism of distal femur injury is typically an indirect force such as varus stress that causes avulsion of the fragment from partial detachment of the proximal lateral collateral ligament, often resulting in no displacement of the epiphysis. Alternatively, open injuries that abrade or skeletonize the area around the physis or loss of a peripheral portion of the physis, such as occurs from lawnmower injuries or motor vehicle trauma, and burns around the physis are other possible mechanisms. This injury is not included in many large series of physeal fractures of the femur but it is exceedingly rare. In one series of 29,878 children’s fractures, only 36 were identified as Salter–Harris VI injuries.³²

Classification by Displacement

Several authors have evaluated direction and magnitude of displacement to predict final outcome.^2,37,50,83 Direction of displacement may guide treatment but does not predict the frequency of poor outcomes.^2,37,80 Anterior displacement of the epiphysis, or apex posterior angulation, resulting from violent hyperextension of the knee is associated with an increased risk of neurovascular damage.^21,80 Peroneal nerve injury may occur with significant medial or lateral displacement of the epiphysis. Otherwise, direction of displacement has not been shown to correlate with other complications such as angular deformity, growth disturbance, or loss of motion.

By contrast, the magnitude of displacement has been shown to be predictive of complications.^2,37,83 The critical amount of displacement that is associated with worsening outcomes varies but, generally, displaced fractures of all S-H types are more likely to develop complications compared to nondisplaced fractures. In one study, fractures with displacement of greater than 50% of the transverse diameter of the distal femoral metaphysis on either radiographic view were more likely to develop growth complications compared to less displaced fractures.⁸³ Others have determined that displacement of more than one-third of bone width correlates with more frequent complications.^2,37,50,83 Fractures without bony contact between the fragments and those with metaphyseal comminution,³⁷ both radiographic indicators of high-energy trauma, have also been correlated with an increased risk of complications.

Classification by Age

Age at the time of injury also correlates with the frequency and severity of complications.⁷¹ Distal femoral epiphyseal fractures in children aged 2 to 11 years typically result from high-energy mechanisms and have a poorer prognosis compared to fractures in children younger than 2 years of age or older than 11 years.^24,71 Separations of the distal femoral epiphysis before the age of 2 years generally have satisfactory outcomes,^71,83 possibly because epiphyseal undulations and the central peak are not as prominent in infants (Fig. 28-11A), allowing fractures to occur with less force and less damage to germinal cells and their blood supply.⁶¹ In adolescents, low-energy sports injuries are the most frequent cause of epiphyseal separation. Because children in this age group have little growth remaining, the consequences of growth disturbance, should this complication occur, are often trivial. In juveniles and adolescents, the fracture may pass through the central prominence and lead to central growth arrest because of interference with vascularity in this region or because of the fracture plane exiting and reentering the central physis (Fig. 28-11B).^61,71,81

Outcome Measures of Fractures of the Distal Femoral Physis

In the largest published series^2,24,83 outcomes of distal femoral physeal fractures are determined by clinical assessment and radiographic parameters at follow-up. The primary clinical factors are the resulting neurovascular status of the affected limb and the range of motion of the knee. Secondarily, knee stability is assessed by subjective reporting of symptoms of instability and objective clinical stress testing of the knee ligaments. No study reported knee scores or the results of instrumented tests of knee ligament laxity.

Radiographic assessment of the injured limb is utilized in most studies to assess fracture healing, to identify physeal bar formation, to measure angular deformity about the knee, and to assess for leg-length discrepancies that may result from a growth disturbance. Fracture healing is determined subjectively by identifying fracture line bridging as well as clinical signs of healing. Physeal bar formation may be identified on plain radiographs but also is assessed by MRI or CT scan. Angular deformity is determined by measuring angulation of the fracture fragments or the tibiofemoral angle. Although limb-length discrepancy may be determined clinically, bilateral lower extremity scanograms, obtained by the Bell–Thompson method or by CT scanning, are utilized to assess the true LLD.

SURGICAL AND APPLIED ANATOMY RELATING TO FRACTURES OF THE DISTAL FEMORAL PHYSIS

Ossification and Growth

The epiphysis of the distal femur is the first epiphysis to ossify and is present at birth, appearing as a small round bony structure distal to and in line with the axis of the metaphysis. This epiphyseal ossification center is the only radiographic sign of the larger cartilaginous anlagen of the distal femur. With maturation, the bony distal epiphysis enlarges as the cartilage model ossifies and becomes bicondylar, at times appearing irregular along the distal articular surface as ossification proceeds. From birth to skeletal maturity, the distal femoral physis contributes 70% of the growth of the femur and 37% of the growth of the lower extremity. The annual rate of growth is approximately three-eighths of an inch or 9 to 10 mm. The growth of the distal femur, like the physes of other long bones, ceases at a mean skeletal age of 14 years in girls and 16 years in boys, with a wide range of variability.^1,87

Physeal Anatomy

At birth, the distal femoral physis is flat, or planar, making this physis in infants the least stable compared to other age groups. With maturation, the physis assumes an undulating and more convoluted shape.⁴⁷ By the age of 2 to 3 years, the physis develops an intercondylar groove, or central prominence, as well as sulci that traverse medial and lateral proximal to each condyle. This configuration effectively divides the physis into four quadrants, each with concave surfaces that match the four convex surfaces of the distal femoral metaphysis over a large surface area. The complex physeal geometry and large area of the distal femoral physis contribute to its stability by better resisting shear and torsional forces compared to the smaller, flat physes of infants. The perichondral ring also circumferentially reinforces the physis at its periphery. This structure, combined with the some reinforcement of the physeal periphery by the knee ligaments, provides additional resistance to disruption of the physis.^18,56 During adolescence, however, the perichondrial ring becomes thinner. It is hypothesized that this change contributes to relative weakening of the distal femoral physis, partially explaining the fact that fractures of this physis in adolescents are more frequent and generally occur from lower energy mechanisms compared to children of 2 to 11 years of age.

The irregular configuration of the physis, while contributing to stability, however, also is an important factor in the high incidence of growth disturbance from these fractures. Fracture lines, instead of cleanly traversing the hypertrophic zone and area of provisional calcification, extend through multiple regions of the physis and damage germinal cells regardless of fracture type.⁷¹ In addition, during reduction of displaced fractures, epiphyseal ridges may grind against the metaphyseal projections and further damage cartilage-producing resting cells. Minimizing contact and shear across the physis during reduction is preferable to improve the chances of normal growth after injury. Reductions in the operating room with muscle-relaxing agents, use of traction during reduction, and limiting the number of closed manipulation attempts before converting to open reduction are some techniques that are generally recommended.

Bony Anatomy

Proximal to the medial border of the medial condyle, a small area of the metaphysis of the distal femur widens abruptly, forming the adductor tubercle. The lateral metaphysis, by contrast, flares only minimally at the proximal part of the lateral condyle, forming the lateral epicondyle. The distal femur is divided into two discrete condyles at the level of the knee joint, separated by the intercondylar notch. Nearly the entire distal femur is covered by hyaline cartilage for articulation with the proximal tibia. The anterior, or patellar, surface just proximal to the intercondylar notch, has a shallow midline concavity to accommodate the longitudinal convex ridge of the undersurface of the patella. Posteriorly, the distal femur contacts the tibial cartilage as the knee flexes. The posterior condyles, projections of the femoral condyles posteriorly, contain this cartilage that extends on either side of the intercondylar notch and nearly to the posterior margin of the physis.

The distal femur has well-defined normal anatomical alignment parameters. The mechanical axis of the femur is formed by a line between the centers of the hip and knee joints (Fig. 28-12). A line tangential to the distal surfaces of the two condyles (the joint line) is in approximately 3 degrees of valgus relative to the mechanical axis. The longitudinal axis of the diaphysis of the femur inclines medially in a distal direction at an angle of 6 degrees relative to the mechanical axis and an angle of 9 degrees relative to the distal articular plane.³⁴

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