Does an imbalance of mechanical forces across the knee promote osteoarthritis?
Summary written by Jon Giles, M.D.
Shakoor et al (Arthritis Rheum 46(3):185, 2002) recently reported that end-stage knee osteoarthritis developing after total hip replacement surgery for osteoarthritis tends to occur much more frequently in the contralateral knee than the ipsilateral knee. Thus, of 75 OA patients who required total knee replacement following total hip replacement, 71% of the arthroplasties were in the knee contralateral to the prosthetic hip. Interestingly, this discrepancy was not seen in patients with rheumatoid arthritis wherein only 15% of the knees with end-stage OA were contralateral to the prosthetic hip. These same investigators now evaluated whether higher forces across the knee contralateral to the prosthetic hip (compared to the ipsilateral knee) might explain this discrepancy (Shakoor et al, Arthritis Rheum 48(6):1556, 2003). Dynamic knee joint loading before and 1-2 years after unilateral total hip replacement was performed.
Methods: 50 patients who had undergone gait analysis prior to unilateral hip replacement surgery were identified from the Gait Laboratory database at Rush Medical College. Patients with diagnoses of inflammatory arthropathy, preexisting contralateral hip or any documented knee OA, or any previous lower extremity joint replacement were excluded. 22 of these patients had repeat gait analysis 1-2 years after successful hip replacement surgery.
Gait analyses were performed with a multicamera optoelectronic system combined with a multicomponent force plate to examine motion and forces exerted on the joints of the lower extremities of patients during walking trials. Validated models were used to determine normalized joint reaction forces at the medial and lateral knee compartments.
Results: At the time of the initial gait analysis, the mean age of the 50 subjects was 62 (range 42-74), and mean body mass was 86 kg (range 63-124 kg). BMI calculations were not included. The demographic characteristics of the 22 subjects who had repeat gait analyses after hip replacement were roughly similar with the exception of the proportion of men to women: 29/21 in the preoperative group vs. 16/6 in the postoperative group.
Preoperatively in the 50 subjects tested with gait analysis, peak knee moments (in particular: peak flexion moment, peak extension moment, peak adduction moment, and peak internal rotation moment) and medial compartment load were significantly higher in the contralateral knee than the ipsilateral knee.
From this group, 22 subjects had repeat gait testing a mean of 15 months after total hip replacement surgery. On repeat testing, peak extension moment, peak adduction moment, and medial compartment load continued to be significantly higher in the contralateral than the ipsilateral knee, whereas peak flexion moment and peak internal rotation moment were equivalent in the two knees.
Conclusions: End-stage OA of the knees evolves in a non-random way in patients who have end-stage OA (and arthroplasty) of the hip. The current studies indicate that the knee contralateral to the affected hip is subjected to higher dynamic joint loads than is the ipsilateral knee, and this asymmetric loading persists long after subjects have undergone successful total hip arthroplasty. This higher dynamic load may be a critical risk factor for the subsequent development of OA of the knee.
Editorial Comments: That excessive or deranged mechanical forces across the knee lead to the development of OA has been a long held and popular hypothesis; however, there are few definitive prospective data in humans that prove this. The observation in 2002 by Shakoor et al showing non-random evolution of knee OA associated with end-stage hip OA strongly supports this notion. The current study lends further support by actually measuring forces across the knee during ambulation. The missing piece of the puzzle, however, is the long-term outcome of these patients. None of them had knee OA at the time that either of these gait analyses were performed. The critical question is whether these patients with high forces in the knee contralateral to the affected hip will be more likely to develop OA in that knee compared to the ipsilateral knee, and compared to patients with lower forces across the contralateral knee. Indeed, the authors readily point out that the external knee adduction moments measured in patients in this study tended to be lower than historically reported moments associated with the development of primary knee OA. Nonetheless, this study represents an important observation and suggests that an earlier and more prominent role for gait manipulation (via behavioral gait modification or engineered devices) as a possible deterrent against the progression of lower extremity OA Should be investigated further.