Medical grade polyurethanes have been widely promoted for biomedical applications. In particular, the use of polycarbonate-urethanes (PCU) has drawn considerable attention in the orthopaedic device industry as a result of their excellent mechanical properties, biostability and biocompatibility. PCUs have been extensively utilized in vascular grafts, stents and artificial heart valves. Specifically, bionate thermoplastic PCU, commercially produced by DSM PTG (Berkeley, California), has been of great interest in the field of orthopaedics because of its outstanding load-bearing properties and excellent wear resistance. Also, it is characterized by its long-term durability and resistance to hydrolytic degradation making it a good candidate for in-vivo orthopaedic applications. PCUs have been considered for meniscal replacement because of its unique weight-bearing capabilities, ability to withstand intense forces within the knee joint and ease of lubrication due to its hydrophilic nature. In addition, the low frictional properties essential for a meniscal replacement is obtainable with PCUs.

Materials used for this study were a commercial polycarbonate-urethanes, Bionate PCU 80A (B8) and 90A (B9) pellets, and polyethylene continuous strands fibres (PE) obtained from DSM Polymer Technology Group, USA. Some quantity of the B8 and B9 pellets were dried separately in a vacuum oven at 100°C for 14 hours. A custom mould was designed for the production of the mechanical test samples. The quantity of the constituent materials was determined using composite theory known as the Rule of Mixtures.

$E_c=E_m{V}_m+E_f{E}_f$

where V_m and V_f are the volume fraction of the matrix and fibre respectively. Three specimens each of the prepared composites were tested for tensile and compression strength and at a crosshead speed of 12 mm/min using a Zwick/Roell 1484 Material Testing Machine.

The PCUs were not as stiff as their fibre-reinforced composites, which indicate that the stiffness of the PCU composite materials is a function of both the stiffness of the PCU matrix and the interspersed fibres. The tensile moduli of composites of B8 and B9 increased appreciably with PE. An increase of 227% was obtained for the B8 with the incorporation of PE fibres while percentage increase in stiffness for B9 was 148% for PE reinforcement fibres. The compressive modulus dropped with the inclusion of the PE fibres in the B9, a reduction of 55% was recorded while an increment of 4% was obtained with PE added to the B8.

The results from this study demonstrate that the tensile and compressive properties of PCU can be custom-tailored to that of the meniscal tissue by systematically embedding reinforcement fibres into the PCU matrix such that a composite with desirable mechanical properties is obtained. The results of both tensile and compressive results visibly revealed the reinforcing effect of the fibres used in this study. However, additional studies are required to completely describe the PCU composite as a candidate meniscal substitute capable of gaining its full functionality.

Introduction: A secure repair of the subscapularis represents an integral part of any surgery involving the anterior approach to the shoulder. Dysfunction of the subscapularis leads not only to poor functional results but also to anterior joint instability which is potentially untreatable. We have devised a new technique of double row fixation of the subscapularis using two suture anchors.

Aim: To evaluate the biomechanical strength of this double row technique against the established methods of simple suturing and transosseous repair techniques.

Method: Twenty matched pairs of human cadaveric shoulders were allocated into 3 groups. Group 1 consisted of 10 shoulders repaired with the double row technique. This involved incising the subscapularis along the bicipital groove and a lesser tuberosity osteotomy carried out leaving the subscapularis attached to a thin island of bone. A suture anchor (Twinfix) was then inserted just medial to the osteotomy site and the tendon repaired to bone using two horizontal mattress sutures. A second anchor was inserted laterally to supplement the repair with two simple suture knots. The remaining 10 contralateral shoulders were allocated equally between groups 2 and 3. In group 2, the subscapularis was divided longitudinally 1cm medial to the bicipital groove and repaired with simple interrupted suture knots. In group 3, the subscapularis was incised at its insertion to lesser tuberosity and the tendon repaired to the osteotomy site by multiple transosseous sutures through drill holes in the anterior humeral cortex.

The suture material used in all three groups was identical and consisted of an ultra high molecular weight poly-ethylene suture (Ultrabraid). To simulate the direction of pull of the subscapularis, the testing block was tilted 45 degrees while a vertically applied distraction force was applied. A custom made jig was used to measure the amount of displacement in response to a gradually applied load. All specimens were tested to failure. The mode of failure of each fixational construct was recorded.

Results: The load to failure was found to be significantly higher in the double row repair technique compared to simple suturing and transosseous methods. Simple suturing failed by suture cutting out of soft tissue and tranosseous repair failed by a combination of the suture cutting out through bone and soft tissue.

Conclusion: This new double row technique is simple to perform and preliminary biomechanical testing has shown this to be superior in terms of fixational strength compared to established methods. Additional advantages of this technique which have not been taken into account in this in vitro study include non violation of the subscapularis tendon with bone to bone healing.