Superior bone fixation of conical compared with hemispherical trapezial cup design: an experimental radiostereometry study

PURPOSE

The most used cup designs for trapeziometacarpal (TMC) arthroplasty are of hemispherical and conical geometrical shape. Using a validated pig bone model, we compared the bone fixation using radiostereometry (RSA).

METHODS

Twenty saddle-shaped pig forefoot bones were prepared with insertion of bone markers and reaming. Hemispherical Type T cups (Beznoska, Kladno, Czech Republic) (N = 10) and conical Moovis cups (Stryker, Pusignan, France) (N = 10) of 9-mm diameter were inserted press-fit. The bones were fixed in cement blocks for stability, and the cups were loaded in a motorized test stand. First, a low-pressure cyclic load test (0-150N) with 130 compression cycles was performed. Next, a push-in test of progressive loads with 50N increments (range: 150-900N) was applied until a visual change in cup position appeared. Cup migration was evaluated with RSA after every new load application. Cup failure was defined as total translation > 0.5 mm between two load applications.

RESULTS

Both cup types tolerated a compression load of 450 N without failure. Beyond this load level, the total translation cup migration of mean 0.20 mm (95% CI 0.11; 0.30) for the Type T group was higher than mean 0.10 mm (95% CI 0.06; 0.15) of the Moovis group (p = 0.046). The Hazard ratio for failure was 0.52 (95% CI 0.12; 2.17) (p = 0.37), indicating that the risk of failure was two-fold higher in the Type T group.

CONCLUSION

We conclude that conical TMC cups have superior fixation as compared to hemispherical cups above a loading level of 450 N, which correspond to a 3.8 kg tip-pinch. In a clinical perspective, based on the fixation strength of both cup types, it seems safe to allow light-load activities of daily living such as buttoning a shirt and using a key shortly after surgery and until sufficient osseointegration is achieved.

Joint replacement for trapeziometacarpal osteoarthritis: implants and outcomes

For thumb carpometacarpal osteoarthritis, trapeziectomy provides generally good results with reduction of pain and restoration of thumb mobility, but shortening of the thumb may at least in theory lead to reduction in grip and pinch strength. Furthermore, instability of the base of the first metacarpal and contact with the scaphoid may occur regardless of the several described techniques with ligament reconstruction and tendon interposition. Thus, for years alternatives to trapeziectomy have been explored and reported with disputable success. Often the road from idea to implantation in patients seems to have been short, and high complication rates and failure of new implant designs have been fairly common. This has led to the general impression that the implants do not provide sufficient benefits compared with simple trapeziectomy. More recent uncemented total joint replacement designs have shown rapid rehabilitation, reduction of pain, restoration of grip strength and movement, and much improved long-term implant survival. These factors may make total joint replacement a realistic alternative to trapeziectomy. Still, patient selection has to be improved, as the benefits may be small in the low activity elderly patient.

Early Cup Loosening After Metal-on-Metal Total Joint Replacement of the Trapeziometacarpal Joint: A Retrieval Study

PURPOSE

To investigate the possible mechanisms behind early cup loosening in a metal-on-metal trapeziometacarpal joint replacement.

METHODS

The trapezia from 5 female patients were removed as part of a salvage procedure after a median of 22 months (range, 7-43 months) after implantation. Three osteoarthritic patients with symptomatic cup loosening and 2 with instability had a median age of 62 years (range, 59-65 years) at primary surgery. The trapezia with cups in situ were preserved and processed for histomorphometry. Studies with laser ablation inductively coupled plasma mass spectrometry and scanning electron microscopy with energy dispersive x-ray spectroscopy were also performed on 2 of the specimens.

RESULTS

In all 5 specimens, osteolytic lesions undermined the cups and were also seen at the cup edges, completely surrounding the loose cups. Large amounts of dark particular material were seen in the periprosthetic tissues, mostly internalized by macrophages. The presence of chrome and cobalt in these regions was confirmed. Four of the 5 cups showed marked or complete loss of hydroxyapatite.

CONCLUSIONS

We have found several possible explanations for the poor performance of this cup, including its cannulated design and metal-on-metal bearing. The changes seen are early and advanced, raising serious concerns about the implant and particularly the articulation.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic V.

High revision rates with the metal-on-metal Motec carpometacarpal joint prosthesis

We retrospectively evaluated a consecutive series of 42 Motec thumb carpometacarpal total joint arthroplasties. The primary endpoint was revision with implant removal and trapeziectomy. At follow-up the disability of the arm shoulder and hand (DASH) score, pain on numerical rating scale at rest and with activity and serum chrome and cobalt concentrations were assessed for both unrevised and revised patients. At a mean follow-up of 26 months, 17 patients had been revised. The 2 year cumulative revision rate was 42% (95% CI, 28-60%). The DASH score and pain scores at rest and with activity were comparable between the patients whose thumbs remained unrevised and those revised. Patients with elevated serum chrome and cobalt levels had significantly higher DASH and pain scores, but elevated levels were not associated with revision. The revision rate in this study is unacceptably high. However, pain and DASH scores after revision are acceptable and comparable with patients with non-revised implants.

LEVEL OF EVIDENCE

IV.