Cyclical loading causes injury in and around the porcine proximal femoral physeal plate: proposed cause of the development of cam deformity in young athletes

Abstract

Background: The repetitive load to which the adolescent athlete's body is exposed during training and competition affects bone growth. In previous studies, abnormalities of the spine and extremities of adolescent athletes have been described on radiographs and this also applies to the hip. The cam deformity of the hip is an extension of the physeal plate and develops during the adolescent athlete's growth. Studies of the porcine spine have shown that the vertebral endplates, apophyseal rings and intervertebral discs are susceptible to both static and repetitive loads. The proximal physeal plate of the porcine femur is susceptible to static loads, but no studies have been performed on its susceptibility to repetitive loads. The purpose of this study was to investigate the susceptibility of the proximal porcine femur to repetitive loads.

Methods: Descriptive laboratory study. Seven proximal femurs from four young (5 months) pigs were loaded repetitively (50,000 cycles) using a previously developed model. Three were loaded vertically, three antero-superiorly and one was used as a control. All femurs were examined macroscopically, histologically and with MRI after loading.

Results: No macroscopic injuries were detected on any of the femurs after loading. Fluid redistribution was seen in all femurs on MRI compared with the unloaded control. Injuries were seen in all loaded femurs on microscopic examination of histological samples. Injuries, perpendicularly to the physeal plate and fractures adjacent to the plate, were seen in the vertically loaded specimens. In the antero-superiorly loaded specimen, the injury in the growth plate was parallel to the plate.

Conclusion: Repeated loading of the young porcine hip leads to histological injuries in and adjacent to the physeal plate. These injuries are likely to cause growth disturbances in the proximal femur. We propose that such injuries may be induced in adolescent athletes and offer a plausible explanation for the development of the cam deformity.

Keywords: Adolescent; Athlete; Biomechanics; Cam; Femoroacetabular impingement; Hip; Physeal plate; Porcine.

Figure 1

Anatomy and blood supply of the physis. The physis has been expanded to visualize the different cellular zones.

Figure 2

Setup for vertical loading of the specimens.

Figure 3

Setup for antero-superior loading of the specimens, simulating 45° of flexion.

Figure 4

The different zones compared on the MRI of the control to the MRI of the loaded specimens. 1 = Superior and adjacent to the physeal line, laterally to the epiphyseal tubercle (Black arrow). 2 = Superior and adjacent to the physeal line, medially to the epiphyseal tubercle. 3 = Inferior and adjacent to the physeal line, laterally to the epiphyseal tubercle. 4 = Inferior and adjacent to the physeal line, medially to the epiphyseal tubercle. 5 = Epiphysis, laterally to the epiphyseal tubercle. 6 = Epiphysis, medially to the epiphyseal tubercle. 7 = Metaphysis, laterally to the epiphyseal tubercle. 8 = Metaphysis, medially to the epiphyseal tubercle.

Figure 5

In the vertically loaded specimens the injuries of the physeal line (black arrows) lay perpendicular to the physeal line, parallel to the cellular columns of the physeal line. On this microscopic picture, a fracture (white arrows) extending from the epiphyseal bone (above) through the physeal line and into metaphyseal bone (below) is seen.

Figure 6

Microscopic photograph of an antero-superiorly loaded specimen. In the antero-superiorly loaded specimens the injuries lay parallel to the physeal line (black arrows).

Figure 7

A microscopic photograph of a vertically loaded specimen. Fractures of the epiphyseal bone (above, black arrows) and metaphyseal bone (below, white arrows) are seen and the injuries in the physeal line (black arrowheads) are aligned parallel with the cellular columns of the physeal line.

Figure 8

A microscopic picture of the epiphyseal tubercle (black arrowhead) in a vertically loaded specimen. The bone density is higher around the tubercle and injuries (black arrow) were often seen in the zone were the bone becomes less dense.

Figure 9

MRI image of the control hip (left) and a vertically loaded hip (right). After loading, difference in signal intensity was seen medially to the epiphyseal tubercle. A lower signal was seen above the epiphyseal line (white arrow) and higher signal below the epiphyseal line (white arrowhead).