Periprosthetic femur fractures are severe complications after hip arthroplasty. There is a high re-operation rate due to malunion, refracture and stem loosening. Fixation is more rigid when screws are used for proximal fixation of the plate instead of cables. Screws penetrating the cement mantle may damage it and induce loosening of the prosthesis stem. The usage of larger diameter drills can prevent cement damage during screw insertion. There is only little loss in pull-out resistance using larger drills. A metal rod (diameter: 13 mm) was cemented into a transparent plastic tube (diameter: 25 mm), leaving a homogeneous cement layer of 6mm. Drills of different diameters (4.3 mm, 4.3 mm + tapping, 4.5 mm, 4.8 mm) were used to implant uni- and bicortical locking screws (all 5mm outer and 4.4mm core diameter) into the cement layer. Locking head screws (LHS: Synthes, Switzerland), periprosthetic locking screws (PPLS: Synthes, Switzerland) and NCB mulitidirectional locking screws (NCB: Zimmer, USA) were used. The onset of cracks was visually monitored during drilling, tapping and screw implantation. Pull-out resistance was measured on each screw. No crack appeared after implantation of any unicortical screw. No cracks appeared after drilling for bicortical screws. Cracks appeared after tapping or inserting bicortical screws (62.5% of the cases). Increasing the drill diameter reduces the risk of cement mantle cracks (to 25%). Bicortical screws had the highest pull-out resistance (median 3015N compared to 1250N for unicortical screws). Screws with a flat tip, smaller flute or double thread showed higher pullout forces. Unicortical screws can be implanted without damaging the cement. Bicortical screws have higher pull out resistance but bear the risk of cement mantle damage. For insertion of bicortical screws a 4.5 mm drill should be considered instead the usual 4.3 mm one. New screws should be developed for unicortical fixation of periprosthetic fractures combining favorable design properties. Further studies should follow to investigate crack formation and loosening after cyclic loading.Hypotheses
The lateral subvastus approach combined with an osteotomy of the tibial tubercle is a recognised, but rarely used approach for total knee replacement (TKR). A total of 32 patients undergoing primary TKR was randomised into two groups, in one of which the lateral subvastus approach combined with a tibial tubercle osteotomy and in the other the medial parapatellar approach were used. The patients were assessed radiologically and clinically using measurement of the range of movement, a visual analogue patient satisfaction score, the Western Ontario McMasters University Osteoarthritis Index and the American Knee Society score. Four patients were lost to the complete follow-up at two years. At two years there were no significant differences between the groups in any of the parameters for clinical outcome. In the lateral approach group there was one complication due to displacement of the tibial tubercle osteotomy and two osteotomies took more than six months to unite. In the medial approach group, one patient had a partial tear of the quadriceps. There was a significantly greater incidence of lateral patellar subluxation in the medial approach group (3 of 12) compared with the lateral approach group (0 of 16) (p = 0.034), but without any apparent clinical detriment. We conclude that the lateral approach with tibial tubercle osteotomy is a safe technique with an outcome comparable with that of the medial parapatellar approach for TKR, but the increased surgical time and its specific complications do not support its routine use. It would seem to be more appropriate to reserve this technique for patients in whom problems with patellar tracking are anticipated.
