Dislocation of rotating hinge prostheses due to implant's breakage or fatigue failures is occasionally observed in patients following total knee arthroplasty for tumoral indications and revisions total knee arthroplasty [, –].
We performed a biomechanical analysis using a custom made biomechanical apparatus on a test bench. Therefore, the lengths and tapers of the peg of six different rotating hinge knee implants (Limb Preservation System—LPS/M.B.T. (DePuy, Warsaw, IN); S-ROM Noiles (DePuy), Global Modular Resection System—GMRS (Stryker, Mahwah, NJ), RT-Plus (Plus Orthopedics, Mödling, Austria), and NexGen (Zimmer, Kiel, Germany)) were determined with a standard calliper rule (, ). The Zimmer NexGen was tested twice, one time with the thinnest and one time with the thickest polyethylene inlay available because the length of the peg varies with the thickness of the polyethylene inlay.
The results of the biomechanical analysis showed that the design of the peg plays a major role in the stability of a rotating hinge device. We conclude that rotating hinge prostheses with shorter and markedly tapered pegs have the highest angular laxity at any given amount of distraction, and they may become unstable under conditions of mild joint distraction, theoretically. Prosthetic designs with longer, cylindrical pegs might be useful in patients with severe articular compromise because the intrinsic design of such devices allows less tilting under mild joint distraction. Nevertheless, none of the implants allowed dislocation until at least 25mm of distraction. Furthermore, a clinical evaluation is indicated to verify this recommendation.
Kabo et al.  and Harrison Jr. et al.  tested the rotational stability of a rotating hinge device and demonstrated the importance of the soft tissues and the newly formed periprosthetic scar, to protect the prosthesis from excessive rotational stresses, especially when using devices without a rotational stop.
Using a custom made biomechanical apparatus, the current study demonstrated that rotating hinge designs with long, cylindrical, and central rotational stems (GMRS, NexGen, and RT-Plus) were superior to implant designs with shorter and/or more tapered rotational stems in the theoretical setting. The LPS/M.B.T. and S-ROM Noiles implants require at least 26mm and 27mm, respectively, of distraction to dislocate. In contrast, the GMRS, the NexGen (with a 12mm polyethylene inlay), and the RT-Plus devices with truly cylindrical, nontapered central rotational stems required 38mm, 36mm, and 30mm of distraction to dislocate. The NexGen rotating hinge knee with a 26mm polyethylene inlay dislocated at 42mm of distraction. The implants with cylindrical, nontapered central rotational stems also had the lowest tilting angles at any given amount of distraction until dislocation, while the S-ROM Noiles implant showed the highest angular laxity throughout the biomechanical analysis (). One main limitation of the study was the manual pressure, which was used to generate the lateral tilting of the central rotational stem within the tibial rotational cylinder and was not standardized by using a loaded pulley system. Therefore, each observer influenced the measurements with his physical strength, similar to the study of Ward et al. Furthermore, in clinical setting the forces transmitted to the prostheses are influenced by many variables such as patient's age, height, weight, length of the lower extremities, muscle strength, cementation technique of the implant, and the patient's daily habits which were not minded in the current study. Nevertheless, determining the interitem correlation matrix revealed high interobserver agreement and could therefore diminish this bias.
Modular knee prostheses with a rotating hinge articulation are used for reconstruction following tumor resections around the knee, complex primary knee arthroplasty, and revision total knee arthroplasties. Such devices provide a stable reconstruction of the knee when the intrinsic soft-tissue stability had been lost as a result of surgical intervention [–].
Purpose. Rotating hinge knee prostheses should provide a stable situation following reconstruction. We performed a biomechanical analysis to establish the association between design of the central rotational stem (peg) and implant's stability, in a theoretical setting. Methods. Six different rotating hinge designs were tested, and three observers performed two different measurements with a custom made biomechanical apparatus and laterally directed pressure. The aim was to assign the degree of tilting of the peg within the vertical post-in channel by extending the distraction as well as the maximum amount of distraction before the peg's dislocation. An intraclass-correlation coefficient (ICC) was calculated to determine the observer's reliability. Results. Implant designs with cylindrical pegs of different lengths were superior to implant designs with conical or other shaped pegs concerning stability and maximum amount of distraction before dislocation, showing steep rising distraction-angular displacement curves. The ICC at 15mm and 25mm of distraction revealed high interobserver reliability (P Conclusion. The biomechanical analysis showed that rotating hinge prostheses with long and cylindrical pegs have the highest stability at any given amount of distraction. Designs with shorter and markedly tapered pegs may become unstable under conditions of mild joint distraction which has to be proven in future in vivo investigations.