Waris E (1), Ashammakhi N (2), Lehtimäki M (3), Tulamo R-M (4), Kellomäki M (5), Törmälä P (5), Santavirta S (6), Konttinen YT (1).

1. Institute of Biomedicine/ Anatomy, Biomedicum Helsinki, University of Helsinki, Finland;
2. Division of Plastic Surgery, Department of Surgery, Oulu University Hospital;
3. Department of Rheumatoid Surgery and Orthopedics, Tampere University Hospital;
4. Department of Clinical Veterinary Sciences, Faculty of Veterinary Medicine, University of Helsinki;
5. Institute of Biomaterials, Tampere University of Technology;
6. Department of Orthopaedics and Traumatology, Helsinki University Central Hospital.

Silicone interposition arthroplasty is the most commonly used method for reconstruction of metacarpophalangeal (MCP) joints. Swanson silicone arthroplasty restores hand function in appropriately selected patients. However, decreased mobility, recurrent pain and instability become prevalent with long-term follow-up. Reported complications include infection, implant dislocation and fracture [1]. The lack of bone stock may cause a formidable challenge in revision arthroplasty.

Modern tissue engineering technology has led to the development of bioreplaceable scaffolds for reconstruction of small joints of the hand [2]. This experimental study is undertaken to evaluate biological behavior, bioabsorption and biocompatibility of bioreplaceable and Swanson silicone interposition arthroplasties performed in the MCP joints of minipigs using histological, microradiographic, radiographic and range of joint motion (ROM) analyses.

Materials and methods
Two types of bioabsorbable implants, a cylindrical scaffold made of poly-L/D-lactide (L,D-monomer ratio of 96/4, P(L/D)LA 96/4) and a P(L/D)LA 96/4 scaffold combined with a Polyactive® stem, were used in this study. Raw P(L/D)LA 96/4 (Purac biochem B.V. Gorinchem, Netherlands) was melt-spun into a 4-ply multifilament, knitted to tubular jersey and then rolled to a cylindrical scaffold Ø8 x 3.5 mm in size. The stemmed P(L/D)LA 96/4 implant had an elastomer-like stem made of a copolymer of polyethylene glycol terephalate (PEGT) and polybutylene terephalate (PBT), Polyactive®, having a PEGT/PBT proportion of 70/30 (H.C. Implants B.V., Netherlands). The silicone Swanson finger joint implant (Wright Medical Technology, Inc.) of size 00 was appropriate to meet the anatomical requirements.

18 skeletally mature female minipigs were operated. The operation was performed on both fore hooves under tourniquet control in general anaesthesia. A longitudinal incision was made over the dorsum of the fifth MCP joint. After the joint was exposed, the metacarpal head and the base of proximal phalanx were resected, leaving the collateral ligaments intact. All cartilage was removed to simulate end-stage rheumatoid arthritis. The joint reconstruction was achieved with one of three different implants described above: 1) P(L/D)LA 96/4 scaffold, 2) P(L/D)LA 96/4 scaffold with a Polyactive® stem, or 3) Swanson silicone implant. Resection without any implantation serves as the fourth study group.

Throughout the postoperative course the joint is assessed by radiological and range of motion (ROM) examination. The animals are sacrificed at 12 and 24 weeks as well as at 1 and 3 years to obtain three specimens from each study group. The bone specimens are fixed in a series of ethanol immersions of rising concentrations and embedded in methylmetacrylate. For histological microradiographic and OTC-fluorescence studies, longitudinal sections are cut with a microtome. The axillary lymph nodes are evacuated, and also biopsies of the liver and spleen are performed to demonstrate histologically possible lymphadenopathy and silicone granulomas.

Results and discussion
All 18 minipigs have been operated in autumn 2001. Cylindrical P(L/D)LA 96/4 implant serves as a temporary scaffold for fibrous cell invasion and have shown good biocompatibility during the follow up of 10 weeks. Fluid accumulation and sinus formation was seen in all radiographs at 10 and 26 weeks following Polyactive® stem. Swanson silicone implants were surrounded by fibrous capsule. Silicone wear particles were seen in the interface tissue at 10 weeks and were migrated to regional lymph nodes.

The animals are followed up to three years when the P(L/D)LA 96/4 scaffold should be completely absorbed and replaced by the animals own tissue. The bioreplaceable, biocompatible, P(L/D)LA 96/4 scaffold provides apparently a resilient, functional MCP arthroplasty allowing unlimited use of the joint and avoiding long-term complications and restrictions of current biostable prostheses.

References
[1] Vasenius J et al. Current Orthopaedics 14, 284-289, 2000;
[2] Lehto MUK, Lehtimäki, MY, Paasimaa S, Törmälä, P: PCT Pat. Appl. No. FI96/00035;
[3] Honkanen P et al. Biodegradable PLDLA scaffold prosthesis in the MCP-arthroplasty of rheumatoid patients. SOT 2, 23, 111, 2000