The adult joint-reconstruction community has made great strides in the last 2 decades in understanding what causes aseptic loosening of arthroplasty components. For example, revelations about polyethylene particulate debris has led to the production of highly cross-linked polyethylene, which in turn has lowered wear rates, decreased revision rates, and increased the survivorship of total hip implants (see related OrthoBuzz post). Still, polyethylene debris is only one factor that can lead to aseptic loosening. Another important, yet often overlooked, factor is friction between the impacted acetabular shell and the host bone.
In the October 3, 2018 issue of The Journal, Bergmann et al. report data that help us better understand the “friction factor” in aseptic loosening. The authors implanted specially designed, instrumented acetabular components that measured in vivo friction moments among nine patients while they engaged in >1,400 different activities. The authors found that 124 of those activities led to friction moments >4 Nm—which appears to be the upper limit for facilitating a firm union between the acetabular component and the native socket.
Movements such as muscle stretching in the lunge position, the breaststroke in swimming, 2-legged standing with muscles contracted, and a single-legged stance while moving the contralateral leg were among those that created the highest friction between the implant and the host bone—and that could impede bone ingrowth into the acetabular component and thus contribute to aseptic loosening. The study also highlights the importance of periodic unloading of the prosthetic joint to allow proper synovial lubrication, which helps minimize the effects of high-friction moments. The good news is that the vast majority of activities studied do not appear to result in friction forces above the 4 Nm threshold.
Although these data should be confirmed with other in-vivo instrumented prostheses (assuming there are more patients willing to receive acetabular components capable of delivering telemetric data), they provide practical insight into the real-world forces placed on total hip prostheses after implantation. Such information can be used to counsel patients regarding high-friction and sustained-loading activities to be avoided, and it can help physical therapists and surgeons tailor postoperative regimens that optimize patient recovery while minimizing the risk to implanted prostheses.
Marc Swiontkowski, MD