A comparative study on four screw-plate locking systems in sheep: a clinical and radiological study.

Four mandibular screw-plate locking systems were studied in sheep. Three to four centimetres angular continuity resections of the mandible were performed and the defects bridged with pre-bent angular reconstruction plates fixed with four screws in the body part and three in the ramus. Each type was used four times. Clinical and radiological examinations were carried out 1 and 2 months later. All sheep were able to eat and ruminate normally throughout the postoperative period. Radiology revealed that 6/16 plates and 5/112 screws fractured during the follow-up period. In one type, no fracture occurred. Screw fracture was confined to just one reconstruction system. Six of 16 mandibles showed slight to moderate bone resorption under the plate. The results point to the internal shortcomings of the systems tested.

Fixation with reconstruction plates under critical conditions: the role of screw characteristics.

Mandibular angular ostectomy defects in 12 sheep were bridged with a titanium plate system (THORP), designed according to the screw-plate-locking principle. Two screw designs (hollow and solid) and two rough (plasma-coated, sand-blasted)-surface and one smooth (anodized)-surface structure were tested in a critical two-screw anchorage model. Fixation with two screws only per fragment was successful in only one-third of the cases. The fixation results were superior for screws with a rough surface, either sand-blasted or plasma-coated. The hollow-screw design did not seem to offer special advantages over the solid screws with identical surface structure. In the successful cases, the bone-screw contact was maintained continuously over a period of 8 weeks, and anchorage was even enhanced by reinforcement of the cancellous structures. In the unsuccessful cases, peri-implant bone resorption seemed to be related to jeopardized bone perfusion in the vicinity of the implants. It is presumed that a plating system with locked screws has the advantage that only two screws will hold a relatively short condylar process. This study proved the assumption to be questionable. However, in view of the problems related to prosthetic replacement of the temporomandibular joint, an attempt to salvage the articular portion may still be justified in selected cases, even if the chance of success is as low as 30%.

Biologic anchorage of cruciate ligament prosthesis. Bone ingrowth and fixation of the Gore-Tex ligament in sheep.

The biologic fixation and strength of fixation of the polytetrafluoroethylene (PTFE) Gore-Tex ligament prosthesis was investigated in sheep knees. The device was inserted to replace the anterior cruciate ligament according to the recommended technique. Histological bone tunnel evaluation together with mechanical tensile studies were done at 6, 12, and 18 months. Already at 6 months the pull-out load of the prosthesis exceeded that of the normal ligament, and this finding persisted up to 18 months postoperatively. At 6 months there was marked fibrous tissue ingrowth into the prosthesis, and at 12 months trabecular bone had replaced the fibrous tissue between the interstices of the filaments; at 18 months bone even penetrated into the individual porous fibers of the prosthesis. The intra-articular part of the prosthesis was surrounded and partly invaded by undifferentiated connective tissue, with no recognizable macrophages or other inflammatory cells. In this experiment, the biocompatibility and porosity of the Gore-Tex prosthesis seemed optimal to permit ingrowth from surrounding fibrous and osseous tissues and firm anchorage into the bone tunnels.

Mechanical properties of biodegradable ligament augmentation device of poly(L-lactide) in vitro and in vivo.

To evaluate the mechanical properties of absorbable braided poly(L-lactide) (PLLA) fibre implants, 2.0 and 3.2 mm in diameter, maximum load defined as tensile load carrying capacity, elongation and axial rigidity were investigated after immersion in phosphate-buffered distilled water at 37 degrees C and pH 6.1 and after subcutaneous implantation in rabbit. The results confirm earlier indications that PLLA degrades faster in vivo than in vitro. The non-sterilized 2.0 mm implants lost 69% of initial tensile load carrying capacity in 46 wk in vitro. In vivo the loss of tensile load carrying capacity of the 2.0 and 3.2 mm implants was most marked between 6 to 12 wk. After 48 wk in vivo 2.0 and 3.2 mm implants retained 3 and 4% of initial tensile load carrying capacity respectively. Both in vitro and in vivo, elongation diminished in the same way as the maximum load. In vitro, mean axial rigidity of unsterilized 2.0 mm implants was 64 N during the first 34 wk but fell to 31 N at 46 wk. In vivo the initial mean axial rigidity of the 2.0 and 3.2 mm implants was 29 and 95 N respectively. At 24 wk the mean axial rigidity was 2 N in both implants.

Bridging of mandibular defects with two different reconstruction systems: an experimental study.

To compare two different mandibular reconstruction systems, a 5-cm angular resection was performed in nine adult sheep. The defect was reconstructed using either a classic AO plate or the AO-THORP reconstruction system. The animals were killed 5, 9, and 14 weeks after operation. Serial sections with the plates and screws in place were prepared and studied histologically and with microradiography and fluorescence microscopy. Twenty-six percent of the 2.7-mm AO cortical screws were adequately fixed. The corresponding percentage for AO-THORP hollow screws was 73%. The main advantage of the latter system is the screw-plate locking principle, which allows plate stability to be maintained even if bone is resorbed under the plate. Whether the existence of a screw lumen is an advantage has not yet been established.

Carbon fiber as a prosthetic anterior cruciate ligament. A biochemical and histological analysis in pigs.

Connective tissue ingrowth into a carbon fiber prosthesis inserted as a substitute for the ACL was biochemically quantified in an experimental pig model with a followup of 16 weeks. During this time, the net collagen amount expressed as hydroxyproline increased to only half that of the normal control ligament. Histological studies revealed relatively few fibroblasts capable of producing collagen inside the alloplastic ligament. When these results are considered in conjunction with those of our earlier report of the disappointing mechanical strength of the carbon fiber substitutes, it seems evident that carbon fiber is not a suitable replacement for the ACL, mainly because of its failure to produce a "neoligament" before it disintegrates.

Collagen types I and III and fibronectin in healing anterior cruciate ligament after reconstruction with carbon fibre.

The presence of the connective tissue components fibronectin and collagen types I and III have been demonstrated by immunohistological methods after reconstruction of the anterior cruciate ligament of pigs with carbon fibre. The carbon fibre ligament prosthesis was covered with pediculated fascia lata. During the initial 16-week period, fibronectin was seen both in the surrounding fascial layer and between the carbon fibres of the substitute. Type III collagen was also found, especially in the fascial layer, but collagen was absent from the carbon fibre prosthesis and was seen only in the fascial layer. Results indicate that the tissue in the 'neoligament' after carbon fibre reconstruction consists mostly of granulation tissue with a high amount of fibronectin, and type III collagen from mesenchymal cells without sufficient amount of type I collagen with tensile strength to withstand subsequent mobilization and rehabilitation.

Preliminary results on the use of medetomidine-ketamine combinations in the dog.

The effects of medetomidine and ketamine by subsequent or simultaneous in injections were studied. ECG, arterial blood gas analyses and mean arterial pressure measurements were done periodically during the 60 min monitoring period. Medetomidine and ketamine induced deep sedation that proved useful for minor operations in clinical work.

Reconstruction of anterior cruciate ligament with carbon fiber. An experimental study on pigs.

During a follow-up period of 16 weeks in 18 pigs, macroscopical and histological studies revealed invasion of fibroblasts and mesenchymal cells around the carbon fiber prosthesis inserted to replace the ACL but scarce tissue ingrowth into the prosthesis proper. There was no adherence of the carbon ligament onto the bony channels. The peak tensile strength of the reconstructed ligament was about one third of the paired normal ACL ligament during the 16 week experiment.