Coxa brevis of the hip results in a morphologic change of the proximal femur creating pain and fatigue, leg-length discrepancy, and altered gait. The most common cause is a growth alteration of the proximal femoral physis secondary to ischemic necrosis of the proximal femoral epiphysis. This article describes the Morscher osteotomy—a unique femoral neck-lengthening technique. The outcomes of this successful and predictable technique are resolution of symptoms of fatigue and hip discomfort; the absence of nonunion, infection, or hardware failure; and restoration of normal proximal femoral morphology and biomechanics.
The immature proximal femur is a complex structure with a physis that contributes to the shape and relationship of the femoral head, femoral neck, and greater trochanter. Siffert described the complexity of the proximal femoral physis by separating the growth plate into three distinct zones of activity. The three zones consist of the longitudinal growth plate, the femoral neck isthmic growth area, and the trochanteric growth plate ( Fig. 1 ). Any disturbance of the normal growth and development of this physis will result in morphologic changes of the proximal femur. A significant shape change of the proximal femur negatively impacts the biomechanics of the hip joint. Altered hip biomechanics result in symptoms of muscle fatigue, pain, and a concurrent limp.
Multiple conditions—including infection, trauma, congenital growth deficiencies or congenital hip dislocation, and avascular necrosis of the femoral epiphysis—can affect the growth and development of the proximal femur. The most common cause of proximal femoral growth disturbance is avascular necrosis of the proximal femoral epiphysis from either an unknown cause, Perthes disease, or from the treatment of congenital hip dislocation. Early in the 1900s, both Waldenström and Perthes described morphologic changes of the femoral neck in patients with avascular necrosis of the femoral epiphysis. Siffert noted that the longitudinal growth plate of the proximal femoral physis depends on the nutrient blood vessels from the femoral epiphysis. Therefore, epiphyseal avascular necrosis of the proximal femoral physis causes an alteration of the longitudinal growth of the femoral neck that allows for a relative “greater trochanteric overgrowth.” This results in a foreshortened femoral neck termed coxa brevis ( Fig. 2 ).
Hip biomechanics
A brief review of hip biomechanics during single-leg stance demonstrates the significant effects of coxa brevis on the strength of the hip abductor muscles (gluteus medius and gluteus minimus). During single-leg stance, the hip abductor musculature must generate a force (M) equal or greater than the weight of the body and opposite leg (L) to stabilize the pelvis and to prevent pelvic obliquity or positive Trendelenburg sign. As Figs. 3 and 4 show, as the femoral neck length increases the hip abductor lever arm increases (Ma). With a longer hip abductor lever arm (Ma), the abductor muscles are more efficient and can counter the weight of the body and opposite limb (L x La) with less force.
Coxa brevis does not only cause a decreased lever arm of the hip, but the relative overgrowth of the greater trochanter decreases the normal tension of the gluteus medius muscle. Elftman demonstrated that the normal muscle tension of the gluteus medius approaches 0 as the muscle resting length approaches 60% of its normal resting length. The greater trochanteric transfer during the femoral neck-lengthening corrects the tension of the gluteus medius by placing the proximal tip of the trochanter at the same level as the center of the femoral head.
Radiographic findings of coxa brevis
Radiographic evaluation of coxa brevis consists of a supine anteroposterior (AP) and frog lateral view of the pelvis along with a supine AP pelvis with maximum hip adduction of the involved side. The femoral head should be assessed for its shape and congruency within the acetabulum. The scope of this article is limited to coxa brevis with a spherical femoral head and a congruent joint. Basic radiographic measurements should include the neck-shaft angle (NSA), anatomic medial proximal femoral angle (MPFA) ( Fig. 5 A and B), and the articulotrochanteric distance (ATD) ( Fig. 5 C and D). In 1965, Edgren described the articulotrochanteric distance (ATD) to objectively quantify the relationship of the femoral head to the greater trochanter ( Fig. 5 C and D).
By comparing the NSA and the MPFA, one can determine if the proximal femoral deformity is secondary to a coxa vara, coxa brevis, or a combination of the two deformities. This determination allows the surgeon to choose the proper operative technique to correct the deformity. In pure coxa vara, both the NSA and the MPFA will have abnormal values compared with the normal side. However, the amount of difference from the normal value in both the NSA and the MPFA will be equivalent. This situation requires a subtrochanteric valgus osteotomy of the proximal femur (Wagner osteotomy) to correct both the NSA and the MPFA ( Fig. 6 ).
On the other hand, if the preoperative radiograph demonstrates abnormal NSA and MPFA differences that are not equivalent, then a single osteotomy will not correct all components of the proximal femoral deformity ( Fig. 7 ). A better strategy is to perform either a Wagner valgus osteotomy with greater trochanteric transfer or a Morscher femoral neck-lengthening osteotomy ( Fig. 8 ).
When assessing the radiographs of a patient with suspected coxa brevis, the age and skeletal development influences the above described radiographic parameters. Radiographic findings of coxa brevis in a younger patient (<8 years) can be very subtle and include a foreshortened and widened femoral neck as compared with the contralateral side. These findings establish the presence of a proximal femoral growth arrest ( Fig. 9 ). Therefore, it is mandatory to image both hips to allow comparison of the proximal femoral morphology to identify these subtle radiographic findings.
Also, the ATD radiographic parameter is inconsistent in younger patients due to the proximal cartilaginous portion of the greater trochanter in patients less than 10 years of age. This creates an apparent ATD versus a true ATD measurement ( Fig. 10 ). This inconsistent radiographic parameter makes diagnosis and determination of treatment efficacy in coxa brevis difficult in younger patients. The ATD should be used in patients 10 years of age and older.
To finalize the radiographic assessment of a patient with coxa brevis, bilateral long-standing radiographs should be performed to determine the amount leg-length discrepancy (LLD) that is present. Long-standing radiographs usually demonstrate LLD of variable severity. This LLD is usually in the range of 1.5 to 3.0 cm ( Fig. 11 ).
Radiographic findings of coxa brevis
Radiographic evaluation of coxa brevis consists of a supine anteroposterior (AP) and frog lateral view of the pelvis along with a supine AP pelvis with maximum hip adduction of the involved side. The femoral head should be assessed for its shape and congruency within the acetabulum. The scope of this article is limited to coxa brevis with a spherical femoral head and a congruent joint. Basic radiographic measurements should include the neck-shaft angle (NSA), anatomic medial proximal femoral angle (MPFA) ( Fig. 5 A and B), and the articulotrochanteric distance (ATD) ( Fig. 5 C and D). In 1965, Edgren described the articulotrochanteric distance (ATD) to objectively quantify the relationship of the femoral head to the greater trochanter ( Fig. 5 C and D).
