26.1.1 Meuli Wrist Arthroplasty

Hans Christoph Meuli from Berne, Switzerland developed and introduced three versions bearing his name starting in 1973. ⁴ The first version was intended for cemented fixation. It had a ball-and-socket articulation, with the ball (made out of polyester at first, then changed to ultra-high molecular weight polyethylene [UHMWPE]) on the proximal side. The proximal and distal components had two prongs, each made of Protasul 10 (a titanium alloy). Initial problems in his series of 41 patients included polyester synovitis, dislocations, technical errors, stem breakage, infection, and ulnar deviation. Modifications based upon these problems led to the final version in 1984 which was an uncemented implant. The components were made of corundum rough-blasted Protasul-100 (Ti6Al7Nb), including the ball, which was separately coated with titanium nitride for wear resistance. The UHMW polyethylene was inserted in the cup in the distal component (inverse articulation) (Fig. 26.1a, b).

Meuli’s own results published with the Meuli III (38 wrists) were satisfactory, with adequate pain reduction, a mean total flexion-extension range of motion (ROM) of 90 degrees and unchanged grip strength. Eight implants needed revision due to loosening during the average follow-up of 5.5 (range 3–9.5) years. ⁵ In a more recent report from Strunk and Bracker, ⁶ 15 Meuli III implanted in patients with rheumatoid arthritis (RA) fared well subjectively after 9 years, but only one was radiologically firmly attached to bone. ⁶ Although the final Meuli III design had some interesting features (including choice of bearing metal and ball-and-socket articulation), the two-prong fixation, the reverse articular concept, and the use of titanium in the articulation with a thin UHMWPE liner led to the failure of the whole concept and the withdrawal of the implant.

26.2.1 Fixation

Rough-blasted titanium alloy (Ti6Al4V) was chosen as the bearing metal due to its excellent uncemented fixation properties as compared to cobalt-chrome-molybdenum (Co-Cr-Mo) or stainless steel. ^{7 , 8} The implant was screw-shaped to increase the surface for fixation and ease the insertion. The fixation was limited to the distal radius (sparing the distal radioulnar joint [DRUJ]) and the capitate in the carpus. Different surface modifications to enhance bone ingrowth were tested in rabbits; eventually we selected a resorbable 15-µm thick calcium-phosphate coating (Bonit, DOT Medical Solutions Laboratories Gmbh). ^{9 , 10}

26.2.2 Articulation

Following wear tests we rejected the use of a titanium-alloy ball coated with titan-niobium; instead we chose a highly polished 18-mm ball-and-socket metal-on-metal (MoM) articulation made of Co-Cr-Mo. It has been shown to have a very low wear rate and excellent performance in hip arthroplasties ¹¹ and has relatively low bulk help to keep the implant small. The ball-and-socket provided almost unrestricted mobility and excellent stability. The system was modular enabling tension adjustment with different neck lengths and the potential for revision of the articulation retaining the bone-anchored implant stems (Fig. 26.2).

26.3.1 The Implant

The Motec (Swemac Orthopaedics AB) wrist implant comprises a thicker, threaded proximal stem available in four different lengths (32–50 mm, in 6-mm increments), a longer and thinner, threaded distal stem available in six different lengths (40–75 mm, 5-mm increment) and two different diameters linked with a modular ball-and-socket articulation with highly polished Co-Cr-Mo MoM or Co-Cr-Mo on poly-ether-ether-ketone (PEEK). The MoPEEK is available in 15 mm and MoM in 15 and 18 mm ball diameters. There are four neck lengths on the balls for tension adjustment (Fig. 26.3).

26.3.2 Patient Selection

The prerequisite for a functional wrist arthroplasty is active muscular control around the wrist. Neuromuscular imbalance or fixed malalignment of the wrist are absolute contraindications in our opinion because an unconstrained wrist implant cannot provide joint stability in such cases. Relative contraindications include prior wrist infection or an unreliable patient. Since 2006 all patients scheduled for wrist arthrodesis in our department have been offered wrist arthroplasty; around 90% have chosen arthroplasty. So far 130 Motec have been implanted—93% in patients with noninflammatory arthritis. The mean age at surgery was 53 (range 18–79) years. The indications are shown in Table 26.1.

The patients were included in a prospective follow-up study. We recorded active ranges of motion (AROM), grip and key-pinch strength measured with JAMAR dynamometer (JA 88 Preston, Corp.), a visual analog score (VAS) for pain, the Quick-DASH (Disability of the Arm, Shoulder and Hand [DASH]) and PRWHE as subjective scores, and radiographs were evaluated preoperatively and at yearly follow-up.