PubMed · 11003084
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A S Greenwald. 2000. Current concepts in joint replacement.. https://doi.org/10.3928/0147-7447-20000901-03
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INTRODUCTION: In order to optimise the primary fixation of the cup of the Arpe (Biomet Merck) trapeziometacarpal prosthesis, several geometries have been studied. The mechanical strengths of the primary fixations ensured by cup "with slots", "bladed" and "with crown", have been assessed and compared to the one obtained for the primary anchorage of the Arpe cup. METHOD: For each cup, the strength of the primary fixation has been assessed in torsion (torque along the cup axis) and bending (torque perpendicular to the cup axis). Tests have been performed on prototype cups set up in a vertebral body of lamb cancellous bone. Torque recording allowed the assessment of the maximum strength for each cup type. RESULTS: Arpe and cup "with slots" showed an effective bending strength, respectively due to the three anchorage picks and to the equatorial over-thickness. However, the cup "with crown" demonstrated a better bending strength with a mean torque of pulling out Cbending = 0.89 Nm. In torsion, the three anchorage picks of the Arpe cup did not allow a solid anchorage. For such a loading, the cup "with crown" also showed the best torsion strength with a mean unsealing torque Ctorsion = 0.83 Nm. DISCUSSION: The equatorial over-thickness seems to give good bending and torsion strengths to the "bladed" and "with crown" cups, with a press-fit effect. Replacing the fixation points of the Arpe cup by a crown also allowed the improvement of its torsion strength.
We report the long term outcome of the trapeziometacarpal prosthesis in a personal series of 13 cases with a follow-up ranging from 12 to 17 years. This ball and socket prosthesis with a metal on polyethylene pairing is cemented. Several modifications were performed along the years: such as increased range of motion, better fit between the metacarpal medullary canal and the proximal part of the stem and addition of a versatile, intermediate component in order to re-establish length of the first ray. The status of the metacarpo-phalangeal joint is critical. Natural hyperextension of this joint has been seen in about two-third of th normal individuals. If the metacarpal component become loose, the thumb column become shorter and hyperextension of the metacarpophalangeal joint is followed by an adduction contracture of the metacarpal which produces a shear force component on the cup and finally the loosening of the cup (sequential loosening). Keeping or re-establishing the length of the first ray is the major principle in the trapeziometacarpal prosthesis. On the other hand, metacarpophalangeal arthrodesis is a contraindication for a trapeziometacarpal prosthesis. Trapeziometacarpal prosthesis should only be used as a last resort when there is severe pain. If the dominant hand is involved, intensive use is a contraindication. The trapezium must be large enough to accommodate the cup. The author recommends a posterior approach which preserve the anterior ligaments and allows a proper bone resection and a good positioning of the cup.
Progressive ankylosis of the trapeziometacarpal joint in flexion-adduction with closure of the first web in advanced trapeziometacarpal osteoarthritis gradually leads to compensatory dislocation of the metacarpophalangeal joint with hyperextension in the sagittal plane and abduction in the frontal plane. This deformity of the MP joint, initially reducible, but subsequently irreducible, results in the classical 'Z' deformity of the thumb in the sagittal plane. A less well known 'Z' deformity can also occur in the frontal plane due to distension of the medial collateral ligament. Surgical treatment of trapeziometacarpal osteoarthritis by arthroplasty must correct this secondary deformity of the MP joint to obtain an optimal result. The classical sagittal 'Z' deformity of the thumb can be easily corrected while this deformity is still reductible by releasing the fist metacarpal by tightening the abductor pollicis longus. When it is irreducible, this deformity can only be treated by MP arthrodesis, which contraindicates insertion of the ARPE trapeziometacarpal implant. Correction of the frontal 'Z' deformity of the thumb requires repair of the medial collateral ligament of the MP joint by ligamentorraphy (retightening) or ligamentoplasty.