CORR Insights®: Is Cam Morphology Found in Ancient and Medieval Populations in Addition to Modern Populations?

Where Are We Now?

Femoroacetabular impingement (FAI) results from abnormal contact between the proximal femur and the rim of the acetabulum. The most common morphology found in patients with symptomatic FAI is the cam morphology, which is characterized by an aspherical shape at the femoral-head neck junction. It can be primary (idiopathic) or secondary to known pathological processes such as slipped capital femoral epiphysis (SCFE) [2].

Studies have disagreed as to whether the cam morphology is a product of modern activities or whether it was present in historical or even ancient populations. Some studies have found evidence of cam morphology in ancient and older 20^th century populations [8, 15], while another study examining the osteology from an indigenous North American population from approximately 1000 years ago did not find evidence of cam morphology, suggesting it may be a modern phenomenon [10].

Here, Musielak and colleagues [11] present a radiological study using osteology specimens examining the prevalence of cam morphology among two different ancient populations (Neolithic population from Iran and Medieval population from Poland) and one modern population (Australian aborigines) [11]. The authors found that cam morphology was present across all three populations at comparable levels of frequency (between 3% to 7%), and male specimens were more likely to have cam morphology relative to female specimens, which is supportive of prior work [8, 15] suggesting that cam morphology is not purely a modern phenomenon.

The prevalence of cam morphology in modern populations appears to be more common than it is in the populations studied by Musielak et al. [11], with a range between 5% to 40% in asymptomatic volunteers with variation among the population studied, measurement technique (plain radiography, cross-sectional imaging), and sex, suggesting that modern lifestyles may exacerbate its development [6, 13].

Based on these discoveries, cam development is likely multifactorial and includes both biological/genetic contributions, as shown by the sex-based differences across a variety of different populations, and activity-related contributions, as illustrated by the different incidence rates across different historical populations compared to modern populations.

Where Do We Need To Go?

Recent studies [1, 3, 6, 12, 13] suggest a multifactorial origin to cam morphology, and further studies are needed to identify the relative contributions of these risk factors. One commonly associated risk factor is physical activity during skeletal immaturity. Cam morphology is thought to originate as an adaptive response to stresses across the capital femoral epiphysis during skeletal maturation, and participation in vigorous sporting activities prior to growth plate closure is thought to be one of the primary sources of cam morphology in modern populations [12]. For example, cam morphology is found more commonly after physeal closure and has a low prevalence in skeletally immature patients, suggesting that it develops around the time of growth plate closure [3]. Participation in youth athletics may contribute to these changes, for instance, an increased prevalence of anterosuperior flattening of the femoral head-neck junction developed over a mean 2-year period in adolescent soccer players close to skeletal maturity [1]. Additionally, increased development of cam morphology was seen in youth hockey players on MRI, most frequently developing at the time of final growth spurt for the capital femoral epiphysis between ages 13 to 16 [7]. While there appears to be an association of youth sports around the time of skeletal maturity with cam deformity, little work has been done to study ways to reduce the stresses across the hip by altering the intensity or the components of sports participation in an attempt to reduce proximal femoral remodeling. Additionally, the role of early surgical or nonsurgical intervention to change the developmental trajectory of these deformities remains unclear.

Another risk factor is developmental pathology of the capital femoral epiphyseal region, such as clinical or subclinical SCFE. Subtle SCFE deformities have been observed in the setting of cam FAI, suggesting that this may also contribute to the development of cam morphology seen at skeletal maturity [2]. Additionally, differences in bony morphology of the epiphyseal tubercle relative to the metaphyseal fossa may affect the relative stability of the growth plate during development, and compensatory mechanisms such as increased epiphy-seal peripheral cupping may ultimately contribute to the development of cam morphology [9].

Another important area of study is the relative contribution of these deformities to future osteoarthritis development. Cam morphology is commonly found in older patients who participated in athletic activities during their lifetime, but this is not necessarily associated with osteoarthritis development, suggesting that further knowledge about the role of cam morphology in osteoarthritis progression is needed to identify the subset of patients who will develop symptomatic osteoarthritis [7]. While biomechanical forces across the growth plate appear to contribute to cam morphologies, they don’t explain their development in all situations. This leaves open the possibility of a biological contribution or genetic differences in structural morphology of the hip as a contribution to cam development and progression to osteoarthritis [10], which future studies ought to investigate.

How Do We Get There?

Large, cross-sectional, prospective, population-based studies have a strong precedent in osteoarthritis research and would be the most influential in defining the origin and natural history of cam morphology in modern populations [5]. Smaller studies have already provided some insight into the origins of cam development, but more research is needed across a diverse range of ages, sexes, geographical distributions, and activity demands including both sedentary patients to competitive athletes. Radiological imaging of the capital femoral epiphysis in both non-sports participants and sports participants from the time of skeletal immaturity until after skeletal maturity will provide insight into the process of cam development and its association with athletic activities or pathological processes of the growth plate. Additionally, these studies may shed light on the populations of patients who may benefit from early surgical or nonsurgical interventions. For instance, loss of hip internal rotation secondary to cam morphology was associated with increased risk of early radiological degenerative changes in adolescent patients [14]. Further insight into the biological development of cam morphology and the progression of osteoarthritis in the setting of FAI would also be beneficial to understanding the underlying pathological mechanisms. For instance, in vivo models that recapitulate cam morphology development provide a controlled setting to understand the relative contributions of biological factors and biomechanical factors that lead to cam development and intra-articular injury [16]. Additionally, human retrieval studies using biological tissues such as peripheral blood, synovial tissue, and cartilage tissue in affected patients may provide further insights into the development of FAI [4] and osteoarthritis progression.