Loading capacity of dynamic knee spacers: a comparison between hand-moulded and COPAL spacers

Abstract

BackgroundThe two-stage revision protocol represents the current gold standard for treating infected total knee replacement implants. Allowing early mobility with weight-bearing between staged procedures will enable early restoration to knee function. So, the mechanical performance of knee spacers is a key issue. Commercially available moulds are often used as they are easy to prepare and produce smoother surfaces of the articulating parts. However, they are costly, and only for single use. A cost-effective alternative is the surgeon-made hand-moulded spacers. In this study, we wanted to determine how the hand-moulded spacers will compare biomechanically with the commercially available COPAL spacers.MethodsSeven cadaveric knees were implanted with knee spacers fabricated using COPAL knee moulds. The same surgeon implanted eight cadaveric knees with hand-moulded spacers. In the first test protocol, an axial load was applied at 200 mm/min till failure. In the second test protocol, the knees were cyclically loaded in five steps of 1000 cycles each from 30-400 N, 30-600 N, 30-800 N, 30-1000 N, 30-1200 N at 1.5 Hz.ResultsCOPAL knee spacers demonstrated a maximum load and mean stiffness of 5202 (± 486.9) N and 1098 (± 201.5) N/mm respectively. The hand-moulded knee spacers demonstrated a mean stiffness of 4509 (± 1092.6) N and 1008.7 (± 275.4) N/mm respectively. The maximum axial displacement was 1.19 ± 0.57 mm and 0.89 ± 0.30 mm for specimens implanted with COPAL knee spacers and hand-moulded spacers respectively. The differences between COPAL and hand-moulded knee spacers were not statistically different.ConclusionsOur study demonstrated that dynamic knee spacers may be able to withstand more than the touch-down load permitted in previous studies, and this may allow more weight-bearing during ambulation. Previous studies have demonstrated that hand-moulded knee spacers have similar advantages to commercially available dynamic spacers with respect to mobility, pain, bone loss, and reinfection rate. Given that ambulation with weight-bearing up to 1200 N is permitted during rehabilitation, it may be more cost-effective to fabricate hand-moulded spacers in revision total knee arthroplasty.

The Inverse Spacer—A Novel, Safe, and Cost-Effective Approach in Routine Procedures for Revision Knee Arthroplasty

“…This novel approach resulted in a Kaplan–Maier failure rate of zero for the spacers and may thus represent an attractive, safe, and cost-effective alternative to other spacers presently used for R-TKA. Detailed characterization of the biomechanical properties of the novel inverse spacers remains the objective of further studies [ 18 ].…”

“…Standardized fabrication of spacers for R-TKA may be of advantage to favor reproducible spacer shapes and features and in order to save operating time and personnel effort. Indeed, there are several studies reporting on pre-fabricated spacer components or molds for standardized production of spacer parts with PMMA bone cement [ 11 , 17 , 18 ]. However, the current, purely hand-made spacer allows both straightforward and efficient molding and optimal adaptation of each spacer component to the existing bone defect until the last minute of polymerization.…”

“…The present study has the following limitations: (i) it contains a limited number of R-TKAs (total of 110), however, it is one of the few studies with a case number clearly exceeding 100 (e.g., [ 4 , 7 , 8 , 9 , 24 ]); (ii) the effective bone loss at the end of the prosthesis-free interval or the long-term clinical outcome after re-operation remains to be determined; (iii) a control group with (an)other spacer(s) is not included; (iv) data cannot be provided on the biomechanical characterization of the spacer [ 18 ] or gait analysis in the prosthesis-free interval, potentially including movement sensors for the spacer.…”

“…Articulating spacers, on the other hand, were primarily developed to improve the mobility in the prosthesis-free interval [ 12 , 13 , 14 ] and therefore to prevent joint stiffness and related complications [ 15 , 16 ]. There are numerous designs of different articulating and non-articulating spacers, including completely hand-made, mold-supported, commercially preformed/customized shapes or even surgical approaches based on removed prostheses following sterilization [ 16 , 17 , 18 ], and various combinations of the above. Contrary to initial expectations based on improved knee motion after re-implantation, static and dynamic spacers did not significantly differ in pain, functional scores, bone loss, or re-infection rates, leading to the suggestion to “consider both static and articulating spacers in the treatment of infection following total knee arthroplasty and to tailor treatment on the basis of patient-related factors” [ 7 , 13 , 19 , 20 , 21 ].…”

Comparison of dynamic and static spacers for the treatment of infections following total knee replacement: a systematic review and meta-analysis

“…However, dynamic spacers are more expensive to produce. And they are not as good as static spacers in maintaining joint stability [18,19]. In terms of infection control, some reports suggest that dynamic spacers and static spacers are similarly effective, while others suggest that dynamic spacers are less effective than static spacers in controlling infection [20][21][22].…”