A Cam deformity is a specific bony abnormality of the hip joint that is increasingly recognized as a significant cause of hip pain in active young adults. This condition involves the proximal femur, where the head and neck meet to form the “ball” of the ball-and-socket joint. While the deformity may remain asymptomatic for years, it is frequently identified in individuals experiencing deep hip or groin pain, often related to physical activity.
Defining the Deformity and Its Impact
The Cam deformity is characterized by an abnormal, non-spherical shape at the junction of the femoral head and neck, typically involving the anterosuperior aspect of the femur. Instead of a smooth, perfectly round transition, a bony prominence or “bump” causes a loss of the normal offset between the femoral head and neck. This structural irregularity can often be visualized radiographically, sometimes appearing as a “pistol grip” shape on X-ray images.
This abnormal shape mechanically restricts the hip’s range of motion, particularly during movements involving deep flexion and internal rotation, such as squatting or pivoting. The bony bump makes contact with the rim of the acetabulum (the hip socket), leading to a condition known as Femoroacetabular Impingement (FAI). This abnormal contact creates increased shear forces at the edge of the joint.
Over time, the repetitive friction and shear forces damage the soft tissues within the hip joint. The primary structures affected are the labrum, a ring of cartilage that deepens the socket, and the articular cartilage lining the joint surfaces. This sustained mechanical trauma is thought to be a precursor to the development of early-onset hip osteoarthritis.
Primary Etiological Theories of Development
Scientific understanding suggests that the Cam deformity is a developmental issue, originating during the years of skeletal growth rather than being a congenital malformation present at birth. One major hypothesis centers on the proximal femoral physis, or growth plate, which is the area of growing tissue near the end of the femur. The Cam shape may result from an unrecognized or residual growth disturbance of this physis during childhood.
One specific theory links the deformity to a mild or healed instance of Slipped Capital Femoral Epiphysis (SCFE). SCFE is a disorder where the growth plate weakens and the head of the femur slips backward and downward relative to the neck. Even a subtle slip can result in the characteristic flattening and prominence at the head-neck junction after the growth plate closes.
Another theory focuses on a localized process of apophyseal overgrowth and asymmetric closure of the growth plate. The anterolateral area, which is the site of the typical Cam bump, may remain open and active longer than the posteromedial sections. This uneven closure allows for continued bone formation and an outward extension of bony growth, creating the excess bone that defines the deformity.
The Critical Role of Skeletal Maturity and Loading
The development of the Cam deformity is strongly associated with the interplay between skeletal maturity and high mechanical forces. Cam morphology is acquired primarily during the adolescent years when the proximal femoral growth plate is still open and actively growing. This period is typically between the ages of 12 and 14 years for boys, though it can vary.
Repetitive, high-impact movements, common in certain sports, are believed to act as the primary external trigger during this vulnerable time. Athletes participating in sports requiring frequent, deep hip flexion and internal rotation, such as ice hockey, soccer, and basketball, show a significantly higher prevalence of Cam deformities than their non-athletic peers. Studies have found a dose-response relationship, indicating that a higher volume and frequency of intense sports practice during skeletal growth increases the risk of developing the deformity.
The mechanism involves excessive mechanical stress placed on the open growth plate, particularly the anterolateral portion. Repetitive shear forces in this area stimulate an adaptive bony response, consistent with Wolff’s Law, which describes how bone adapts to mechanical stress. This localized stress leads to osseous overgrowth at the head-neck junction as the body attempts to strengthen the area, ultimately resulting in the bony bump that is the Cam deformity.
Genetic and Inherited Predispositions
Beyond the mechanical factors, evidence suggests that inherent biology and family history contribute to the risk of developing a Cam deformity. Studies comparing patients to their siblings and control groups have identified a familial link. For example, siblings of individuals diagnosed with a Cam deformity have been found to have a relative risk of 2.8 of possessing the same deformity compared to controls.
While no single gene has been definitively identified as the cause, this familial clustering suggests a genetic predisposition may influence the shape of the hip or the way the growth plate responds to stress. Subtle pre-existing differences in the anatomy of the hip, such as a decreased neck-shaft angle or variations in femoral version, may also increase susceptibility. These slight differences in bone alignment could alter the biomechanics of the hip, making the growth plate more vulnerable to the shearing forces from athletic activity. The combination of an inherited, susceptible hip shape and high mechanical loading during adolescence likely creates the highest risk for Cam formation.