Successes and Failures of a Freedomâ„¢ Constrained Cup Used in a Major Salvage Procedure
Background: This is a case report of a 36mm constrained cup (Freedom™, Biomet IN) that performed successfully for 7-years in a salvage case involving a total-femur implanted in a leg already short by 3-4 inches. The goal was to enhance hip motion and stability using a 36mm head instead of the usual 32mm size. Templating indications were for a 50mm cup (Freedom™; Arcom™ liner). The proximal femur inserted in 2008 incorporated the 36mm constrained THA and was anchored distally to bone using the Compress™ fixator. By 2012 the fixator loosened and was replaced by hinged total-knee arthroplasty (TKA). The THA was retained at revision and patient’s clinical follow-up was satisfactory for 4 years. As indicated by Martel radiographic method, the Arcom™ liner showed minimal wear over this period. Radiographs in Feb-2016 showed the cup’s constraint ring had rotated slightly but the patient had no symptoms. By Dec-2016, the patient had experienced three falls and also had heard a popping sound in her hip. At Dec-2016 office visit, radiographs indicated additional rotation of the constraint ring and CT scans showed an eccentric head position contacting the metal shell. At revision, 50% of the Arcom rim was ablated and the remainder present as a loose fragment. Following insertion of new Freedom liner and 36mm head, her follow-up appears satisfactory 10-months later. Her leg shortening remains but she walks to office visits using a cane and doesn’t need the cane at home.
Methods: Retrieved Arcom liner and detached rim fragment were reconstructed, photographed, and then bi-valved for comparison to similarly prepared exemplar liners, one identical to our revision and one with a thicker wall. Details of liner sections were taken from photographs and reconstructed by computer graphics (Canvas Draw-3™). Wear performance over the first 7 years was assessed using the Martel x-ray method.
Results: Inspection of retrieved liner showed a large oval depression in the ablated rim. The contra-rim featured the large Arcom fragment and the underlying liner wall was less than 1mm thick. Comparison to exemplar liners showed that the large fragment had separated along the lower edge of the constraint groove. Exemplars demonstrated a substantial rim buttress spanning 13mm, which had been ablated in our retrieval.
Discussion and Conclusion: Although this was not a high-demand patient, the considerable hip-impingement forces in a flail limb likely levered the head repeatedly against the liner’s constrained rim. Neck impingement was clearly evident in the damaged liner. A subluxing femoral head would also thin the contra-wall, as would backside wear. We do not know if the eccentric ring image in Feb-2016 radiographs depicted failure. The liner may have escaped from the shell’s locking-ring and with activity, ablated the Arcom contours and led to rim fracture. It is also possible that the liner constraint was damaged when the patient fell, thereby allowing the liner to mobilize.
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Noble PC, Durrani SK, Usrey MM (2012) Constrained cups appear incapable of meeting the demands of revision THA. Clin Orthop 470: 1907–1916.
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