Albert J. Pennings

Resorbable materials of poly(l-lactide). VI. Plates and screws for internal fracture fixation

Poly(L-lactide) (PLLA) with an extremely high molecular weight (Mv up to 1 X 10(6)) was synthesized at a low catalyst concentration (0.015 wt%) and temperatures between 100-110 degrees C. Besides good mechanical properties the as-polymerized PLLA exhibited a microporous structure. Plates and screws of this material were used for the treatment of mandibular fractures, both in dogs and in sheep. Bone healing was uneventful and proceeded without callus formation or signs of inflammation. Fracture healing was accompanied by a progressive degradation of the microporous implants of PLLA.

Resorbable materials of poly(l-lactide): VII. In vivo and in vitro degradation

In vivo and in vitro degradation of high molecular weight poly(L-lactide) used for internal bone fixation has been investigated. Within 3 months as-polymerized, microporous PLLA (Mv = 6.8-9.5 X 10(5] exhibited a massive strength-loss (sigma b = 68-75 MPa to sigma b = 4 MPa) and decrease of Mv (90-95%). At week 39, the first signs of resorption were evident (mass-loss 5 wt%). Except for dynamically loaded bone plates no differences between in vivo and in vitro degradation of PLLA were observed. The increase of crystallinity of PLLA upon degradation (up to 83%) is likely to be attributed to recrystallization of tie-chain segments. A more ductile PLLA exhibiting a lower rate of degradation was prepared by extraction of low molecular weight compounds with ethyl acetate.

Ultrahigh-Strength Polyethylene Filaments by Solution Spinning and Hot Drawing

The past decade has seen a rapidly growing interest in highmodulus and high-strength polymeric fibers (see e.g. CIFERBI, WARD 1979). The methods to prepare these filaments are based on cold/ hot drawing (CAPACCI0, WARD 1974), two-stage drawing (CLARK, SCOTT 1974), hydrostatic (GI~SON et al. 1974) and direct (SOUTHERN, POR~ER 1970; TAKAYANAGI et al. 1966) extrusion, elongational flow (PENNINGS et al. 1972; ZWIJNENBURG, PENNINGS 1976) and on the intrinsic rigidity of particular macromolecules in solution (e.g. CIFERRI 1975). High-modulus polyethylene filaments with Youngs moduli of about 70 GPa have been produced by CAPACCI0 and WARD (1974) through drawing. PORTER et al. (1970) performed solid-state extrusion to obtain polyethylene structures with similar mechanical properties. The tensile strength of these materials is usually found below I GPa (CAPACCI0, WARD 1975; KOJIMA, PORTER 1978). Longitudinal crystals of polyethylene with a Youngs modulus of 100-120 GPa and a tensile strength of 3-4 GPa have been obtained from solution by ZWIJNEN~JRG and P~NNINGS (PENNINGS 1977) in a Couette-type apparatus employing a crystallization technique referred to as surface growth. This short communication describes some preliminary results on alternative routes to produce high-strength and high-modulus polyethylene filaments with a uni-axial Youngs modulus of 90 GPa and a tensile strength of 3 GPa. The method is based on a simple solution-spinning and drawing process that can be performed continuously.

HIGH-MOLECULAR-WEIGHT COPOLYMERS OF L-LACTIDE AND EPSILON-CAPROLACTONE AS BIODEGRADABLE ELASTOMERIC IMPLANT MATERIALS

SummaryHigh molecular weight copolymers of L-lactide and ε-caprolactone have been synthesized by ring opening copolymerization with stannous octoate as catalyst. The good mechanical properties of the 50/50 copolymers make it a suitable material for biomedical applications such as nerve guides etc., where degradation of the elastomeric implant is required. In contrast to the frequently used MDI containing polyurethanes, degradation products of the P(LLA-ε-CL) are non toxic. The use of such a material is therefore preferable.

Replacement of the Knee Meniscus by a Porous Polymer Implant A Study in Dogs

Background Meniscectomy will lead to articular cartilage degeneration in the long term. Therefore, the authors developed an implant to replace the native meniscus. Hypothesis The porous polymer meniscus implant develops into a neomeniscus and protects the cartilage from degeneration. Study Design Controlled laboratory study. Methods In a dog model, a porous polymer scaffold with optimal properties for tissue infiltration and regeneration of a neomeniscus was implanted and compared with total meniscectomy. The tissue infiltration and redifferentiation in the scaffold, the stiffness of the scaffold, and the articular cartilage degeneration were evaluated. Results Three months after implantation, the implant was completely filled with fibrovascular tissue. After 6 months, the central areas of the implant contained cartilage-like tissue with abundant collagen type II and proteoglycans in their matrix. The foreignbody reaction remained limited to a few giant cells in the implant. The compression modulus of the implant-tissue construct still differed significantly from that of the native meniscus, even at 6 months. Cartilage degeneration was observed both in the meniscectomy group and in the implant group. Conclusion The improved properties of these polymer implants resulted in a faster tissue infiltration and in phenotypical differentiation into tissue resembling that of the native meniscus. However, the material characteristics of the implant need to be improved to prevent degeneration of the articular cartilage. Clinical Relevance The porous polymer implant developed into a polymer-tissue construct that resembled the native meniscus, and with improved gliding characteristics, this prosthesis might be a promising implant for the replacement of the meniscus.

Poly(L-lactide) implants in repair of defects of the orbital floor: An animal study

Because of the life-long presence of alloplastic, nonresorbable orbital floor implants and the complications of their use mentioned in literature, the use of a resorbable material appears to be preferable in the repair of orbital floor defects. A high-molecular-weight, as-polymerized poly(L-lactide) (PLLA) was used for repair of orbital floor defects of the blowout type in goats. An artificial defect was created in the bony floor of both orbits. Reconstruction of the orbital floor was then carried out using a concave PLLA implant of 0.4-mm thickness. At 3, 6, 12, 19, 26, 52, and 78 weeks postoperatively, one goat was killed. Microscopic examination showed full encapsulation of the implant by connective tissue after 3 weeks. After 6 weeks, resorption and remodeling of the bone at the points of support of the implant could be detected. A differentiation between the sinus and orbital sides of the connective tissue capsule was observed. The orbital side showed a significantly more dense capsule than the antral side, which had a loose appearance. At 19 weeks, a bony plate was progressively being formed, and at 78 weeks, new bone had fully covered the plate on the antral and orbital side. No inflammation or rejection of the PLLA implant was seen.

The fracture process of ultra-high strength polyethylene fibres

The fracture behaviour of ultra-high strength polyethylene fibres has been investigated in dead load tests as well as by electron microscopical observation of the fracture surfaces. It was found that the fracture process in the fibres involves an activation energy of about 60 to 75 kJ mol−1, which implies that the strength is mainly determined by the lateral bond strength between the molecules. Fracture is initiated at surface irregularities, such as kink bands, which leads to the formation of cracks with a fibrillated fracture surface. In this process the individual fibrils are cut through at topological defect regions in such fibrils, containing a relatively high concentration of trapped entanglements and chain ends. The ultimate strength of the polyethylene fibres was found to be inversely proportional to the square root of its diameter. Extrapolation to zero diameter yields a strength of 26 GPa for flawless fibres, which equals the theoretical strength of polyethylene.

Porous implants for knee joint meniscus reconstruction: a preliminary study on the role of pore sizes in ingrowth and differentiation of fibrocartilage.

Implants with four different macropore sizes were implanted in the meniscus of 29 rabbits for assessment of ingrowth and differentiation of fibrocartilage. Implant macropores were 50-90, 90-150, 150-250 and 250-500 mum, the vol.% macropores was 48-55 and total pore volume 84-86 vol.%. Ingrowth was optimal in the two large pore implants whereas the small pore implants partially remained empty up to 1 year post-operative. Capsule formation and the foreign body reaction was severe for the small pore implants whereas this occurred to a lesser extent in the two large pore implants. Fibrocartilage formation, as assessed by morphology and antibody labelling for type I and type II collagen, was observed in a similar way in all implant types. It is concluded that for optimal ingrowth and incorporation of partial or total meniscal prosthesis, macropore sizes must be in the range of 150-500 mum.

HIGHLY CRYSTALLINE AS-POLYMERIZED POLY(L-LACTIDE)

SummaryTwo new lactone polymerization catalysts, tin(II)-bis(2,4-pentanedionato-O,O) (Zn(Acac)2) and zinc-bis(2,2-dimethyl-3,5-heptanedionato-O,O) (Zn(DMH)2), together with tin(II)-2-ethylhexanoate (Sn(Oct)2), were used to study the effect of the catalyst in the L-lactide polymerization on the nascent polymer structure. Using Zn(DMH)2 an ultra high crystalline polymer was obtained with a ΔHm of 100 J.g-1. Independent of the catalyst and the crystallinity the structure of the crystalline polymer was a pure α-structure as revealed by WAXS-measurements. SAXS-measurements showed a long period for both the Sn(Oct)2 and the Zn(DMH)2 polymer of about 33 nm., the latter revealing a more complex highly regular structure with up to fourth-order reflections. SEM micrographs showed a fibrillar like structure for the Zn(DMH)2 polymer.

HIGH-IMPACT STRENGTH AS-POLYMERIZED PLLA

SummaryAs-polymerized poly(L-lactide), (PLLA), has a much higher impact strength than after compression moulding. Several routes have been explored to further increase the impact strength of nascent PLLA. First results on the preparation of composites with carbon fibres, the copolymerization with trimethylene carbonate and ε-caprolactone, the block copolymerization with rubbers and the plasticization with ethyl acetate indicate possibilities to prepare much more impact resistant as-polymerized PLLA materials.