Although the majority of total joint replacements do very well, some do not.
Estimates project that by the year 2030 between two to three million total joint surgeries will be performed in the United States (1,2). Revision surgery, whereby a secondary joint replacement operation is performed, accounts for over ten percent of the annual number of joint replacement procedures. This amounts to a revision burden of two to three hundred thousand cases per year. Considering the complexity of these cases, the cost to the medical system are substantial, accounting for well over six billion dollars annually.
In the case of total hip replacement, for example, the successful outcome of the procedure is dependent upon a solid union between the prosthesis, which is press fit within the femoral canal, and the surrounding bone. New bone formation between the supporting bone and the prosthesis account for the long term stability of the construct. (see figure 1)
In order to achieve a stable construct between the bony substrate and the prosthetic implant, movement between the two is usually restricted to forty to fifty microns (3). (To put this distance in perspective, the diameter of a human hair is about seventy five microns).
Persistent motion above one hundred fifty microns may subsequently result in instability. The reason behind this is based upon the biologic ability of a bony union to form between the prosthesis and the surrounding bone. If motion consistently exceeds one hundred fifty microns, scar formation rather than bone formation is initiated. Thus, with excess motion, the bond formation between prosthesis and bone is considerably weaker.
Subsequently, due to micro-pistoning of the implant, bone erosion advances.
By more accurately monitoring the micro motion of orthopaedic implants, remedial measures may be instituted much earlier than our current technology allows; be it protected weight bearing, appropriate medications, or earlier surgical revision.
Of note, the earliest hint of loosening of a hip prosthesis available on an office x-ray is between one and two millimeters (one and two thousand microns). Unfortunately, significant component loosening is already present by the time this radiographic sign is apparent.
There is presently no technology available to monitor the motion of orthopaedic implants at the micron level, in real time, without use of x- rays. OrthoDx has developed a prototype to do just that.
1 Sloan, JBJS, 2018
2 Kurtz, JBJS, 2007
3 Jasty, et al., Copyright 1997 by The Journal of Bone and Joint Surgery, Incorporated