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New research shows a high-riding patella may contribute to patellofemoral osteoarthritis by shifting pressure onto areas of cartilage that do not normally bear load.
Anterior cruciate ligament (ACL) injuries are debilitating, with the highest incidence among young adults who participate in sports like football and basketball. Despite its success in returning most patients to competitive sports, anterior-cruciate-ligament reconstruction (ACLR) does not reduce the risk of development of premature knee osteoarthritis (OA) after ACL injury. Radiographic studies indicate that 50% of those under 40 years of age develop knee OA 8–12 years after ACL injury, irrespective of conservative or surgical management. Using quantitative MRI techniques, more recent studies suggest that knee cartilage degeneration may occur as early as 6 months after ACLR.
In our study, we used mobile biplane x-ray imaging (fluoroscopy) to measure knee joint motion during walking in individuals who had undergone ACLR and those with healthy knees. A unique feature of mobile x-ray imaging is the ability to keep the knee joint in the capture volume formed by the two (biplane) x-ray beams as movements of the bones (femur, tibia and patella) are measured at high frame rates (up to 1000 x-ray images per second) with sub-millimetre precision.
Fifteen young adults (8 female) aged 18 to 35 years old who had undergone primary unilateral hamstring-tendon autograft ACLR 6–15 months before testing were recruited. Ten healthy young adults (4 female) similar in age and stature with no knee pain or history of lower limb pathology served as the control group. Each participant walked on level ground and on a 10° downward slope at their self-selected speed while biplane x-ray images were recorded for one complete walking cycle (heel strike of one leg to the next heel strike of the same leg). The biplane x-ray images were used together with CT scans of each knee to calculate the full 3D movements (3 rotations plus 3 translations) of the femur, tibia and patella at each time point during the walking cycle. These measurements were obtained for the ACLR knee and the uninjured contralateral knee of each ACLR participant, as well as one healthy knee of each participant in the control group. To facilitate interpretation of our knee kinematic measurements, we also calculated the Insall-Salvati ratio, found by dividing the length of the patellar tendon by the length of the patella.
The ACLR and healthy participants walked at approximately the same speed during both activities (~1.2 m/s and ~0.8 m/s for level and downhill walking, respectively). Compared to healthy controls, the ACLR participants had a higher vertical position of the patella and a higher location of articular contact between the patella and femoral trochlea. The articular contact location on the trochlea was on average 7.6 mm higher during both activities. This higher-than-normal vertical position of the patella was observed in both the ACLR knee and the uninjured contralateral knee and can be explained by a longer-than-normal patellar tendon. The patellar tendon was on average 8.9 mm longer (p < 0.001) in both the ACLR and contralateral knees compared to the healthy knee. The mean Insall-Salvati ratios for the ACLR, contralateral, and healthy knees were respectively 1.22 (range: 1.03–1.50), 1.21 (range: 0.99–1.40), and 1.03 (range: 0.93–1.15) (p < 0.001 for both ACLR vs healthy and contralateral vs healthy). An Insall-Salvati ratio ≥ 1.20 indicates the presence of patellar alta, a clinical condition associated with patellar instability and anterior knee pain. There were no significant differences in patella tendon length between the ACLR and contralateral knees.
A high-riding patella may contribute to patellofemoral OA by shifting pressure onto areas of cartilage that do not normally bear load. An elevated position of the patella may lead to joint overloading as higher and more frequent joint loads are applied to regions of the trochlear cartilage not adequately conditioned to withstand such loads. A high-riding patella may also drive the development of patellofemoral OA by raising patellofemoral joint stress, another form of joint overloading believed to contribute to cartilage degeneration. A higher vertical position of the patella would increase patellofemoral joint stress by reducing the contact area between the patella and trochlea throughout the range of knee movement during walking.
As our measurements were made at a single time point 6–15 months after ACLR, we are unable to establish whether a longer-than-normal patellar tendon that resulted in a high-riding patella existed prior to the ACL injury or resulted from the ACL injury or ACLR surgery. Longitudinal studies are needed to determine the cause of a longer-than-normal patellar tendon in those who have undergone ACLR. Future studies should investigate the association between a high-riding patella, patellar tendon length, and the incidence of ACL injury.
This study was first published on 19 March 2025 in the Journal of Orthopaedic Research.
Marcus Pandy is a Professor of Mechanical and Biomedical Engineering at the University of Melbourne.
Shanyuanye Guan is a Postdoctoral Research Fellow in biomechanics in the Department of Mechanical Engineering at the University of Melbourne.
The statements or opinions expressed in this article reflect the views of the authors and do not necessarily represent the official policy of the AMA, the MJA or InSight+ unless so stated.
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