Computer-simulated TOUCH prosthesis cup malposition and solutions.

INTRODUCTION: Total joint replacement has become significantly more common as a treatment for advanced trapeziometacarpal joint osteoarthritis in recent years. The latest generation of prostheses with dual-mobility designs leads to very good functional results and low rates of loosening and dislocation in the short and medium term. Biomechanical studies showed that central placement and parallel alignment of the cup with respect to the proximal articular surface of the trapezium are crucial for both cup stability and prevention of dislocation. Despite correct positioning of the guidewire, incorrect placement or tilting of the inserted cup may occur, requiring immediate intraoperative revision. METHODS: The existing spherical and conical cup models in sizes 9 mm and 10 mm were transferred to a computer-aided design dataset. Depending on the intraoperative complication (tilting or incorrect placement), the revision options resulting from the various combinations of cup type and size were simulated and analyzed according to the resulting defect area and bony contact area. RESULTS: In well centered cups, a size 9 conical cup could be replaced by a size 9 spherical cup and still be fixed by press-fit. Conversely, a size 9 spherical cup could not be replaced by a size 9 conical cup, but only by a size 10 cup, of whatever shape. When a size 9 conical cup was tilted up to 20°, the best revision option was to resect the sclerotic margin and insert a size 10 conical cup deeper into the cancellous bone, to achieve the largest contact area with the surrounding bone. When a size 9 cup of whatever shape was poorly centered (misplaced with respect to the dorsopalmar or radioulnar line of the trapezium), placement should be corrected using a size 10 cup, combined with autologous bone grafting of the defect. Again, the size 10 conical cup showed the largest bony contact area. CONCLUSION: Our computer-based measurements suggested options for intraoperative cup revision depending on cup shape and size and on type of misalignment with resulting bone defects. These suggestions, however, need to be confirmed in anatomic specimens before introducing them into clinical practice.

Is taper corrosion in modular revision hip stem junctions associated with patient or implant specific factors? A retrieval analysis.

Implant modularity within revision total hip arthroplasty (THA) offers multiple implant configurations and allows surgeons a high intraoperative flexibility to restore functionality to the patients joint, even in complex revision cases. However, a rare but devastating complication for patients, clinicians and manufacturers presenting a breakage of the taper junction between the distal stem and the proximal implant part. Aside from implant and patient specific risk factors, corrosion and fretting at the stem junctions have been associated with taper failure. Whether corrosive processes are a precursor of failure or rather an accompaniment of material fatigue is thereby still unclear. Therefore, this study aims to investigate the incidence of taper corrosion in a collection of 17 retrievals from a single type (MRP-Titan, Peter Brehm GmbH) and on the correlation of taper corrosion to implant and patient specific factors. None of the implants was revised for problems related to the taper junction, corrosion or the implant itself. The modular stem junction of all retrievals was visually rated with respect to corrosion, fretting and surface contamination. Additionally, the stability of taper junctions of retrievals where the proximal part with the neck was still assembled to the stem was determined by measuring the loosening moment of the securing screw and the push-out-force for taper dissociation. There was no difference between the mean push-out-force of the retrievals (14 kN ± 1.2 kN) and new reference samples (12.6 kN ± 0.5 kN). Approximately one third of the investigated retrievals showed considerable taper corrosion. The extent of corrosion increased with time in vivo and contamination of the neck piece, while it decreased with the loosening moment of the locking screw. The parameters femoral head offset, neck piece length, lateralized neck pieces, obesity of patients and septic/aseptic revision were not correlated to taper corrosion. Taper corrosion seems to occur regularly in modular taper junctions and is not necessarily connected to taper failure. A correct assembly of the junction and avoiding taper surface contamination during revision surgery is mandatory.

[Cup Placement in Thumb Carpometacarpal Joint Prostheses: The Role of the Guidewire]. / Pfannenplatzierung bei Daumensattelgelenksprothesen: Die Rolle des Führungsdrahts.

BACKGROUND: The increasing use of thumb carpometacarpal joint prostheses for advanced CMC 1 (carpometacarpal) joint arthritis reflects the success of the latest prosthesis generations, which has been achieved through their improved functional outcomes and lower complication rates. Precise alignment of the prosthesis cup parallel to the proximal joint surface of the trapezium is essential for stability and the prevention of dislocation. This is a challenging surgical step, particularly for surgeons new to this technique. Despite adequate positioning of the guidewire, misplacements of the cup may occur, necessitating intraoperative revision. MATERIAL AND METHODS: This study examined the deviations in cup and guidewire positioning in thumb carpometacarpal joint prosthesis implantations by inexperienced and experienced surgeons through radiological analysis of 65 prostheses. RESULTS: Both inexperienced and experienced surgeons achieved precise guidewire positioning with mean deviations of<2.2°. Inexperienced surgeons showed significantly larger cup deviations in the dorsopalmar and lateral view (7.6±6.1° and 7.3±5.9°) compared with experienced surgeons (3.6±2.7° and 3.6±2.5°; p=0.012, p=0.017). The deviation of the cup position exhibited by inexperienced surgeons tends to be in the direction opposite to the initial guidewire position (p<0.0038). CONCLUSION: The results highlight the current challenges in cup positioning depending on a surgeon's level of experience, questioning the reliability of the current guidewire placement.