Optimal screw diameter for interference fixation in a bone tunnel: a porcine model

Abstract

The study investigates the optimal interference screw dimensions required to secure a tendon graft in a bone tunnel. A standard 8 mm pig flexor-tendon graft was inserted into a standard open-ended 8 mm bone tunnel of a porcine distal femur and secured using either 7 mm, 8 mm or 9 mm diameter metal interference screws (Arthrex Inc, Naples, FL). The construct was tested to failure using a Shimadzu ASG 10KN Universal Material Testing Machine (Shimadzu, Tokyo, Japan). Load and mode of construct failure were recorded for 37 individual constructs. There was no significant difference in the load at failure between the 7 mm screw (192 N; range 151-232) and 8 mm screw (181 N; range 150-212) (p>0.05). There was a significant difference between the 7 mm screw and the 9 mm screw (109 N; range 67-151) (p=0.006) and between the 8 mm screw and the 9 mm screw (p=0.015). When using a 9 mm screw, 100% of the constructs failed by cut out of the graft at the tunnel opening. The 7 mm constructs failed by slippage of the tendon from the bone tunnel in 83% of cases, with only 17% failing by cut out at the tunnel opening. The 8 mm constructs demonstrated a mixture of failure modes, with slippage occurring in 58% of cases, cut out in 38% and failure of the graft substance in one case (4%). In this model, screw diameters equal to or 1 mm less than the tunnel/tendon diameter provides better fixation than using a screw 1 mm larger. The mode of failure differs for each of these screws.

Tenodesis of the long head of biceps brachii: Cyclic testing of five methods of fixation in a porcine model

“…It is well known that the tendon diameter, drill hole diameter, and diameter of the interference screw have a great influence on the fixation strength. 11,16,25 However, in this study, a standardized drill and interference screw diameter were used, because the tendon diameter was relatively constant, at 6.1 6 0.5 mm. Furthermore, recommendations regarding the relationship between the diameters of the tendon size, interference screw, and drill hole are controversial.…”

“…Furthermore, recommendations regarding the relationship between the diameters of the tendon size, interference screw, and drill hole are controversial. 11,16,25 Biomechanical testing was not only performed with a simple load-to-failure protocol. To simulate in vivo conditions during the postoperative rehabilitation period, the specimens were tested during 200 cycles of repetitive loading between 10 and 100 N, similar to recent studies.…”

Contact pressure on ACL hamstring grafts in the bone tunnel with interference screw fixation — Dynamic adaptation under load

“…First and foremost is that mechanical performance has been reported to be inferior to other fixation techniques, such as cortical fixation or metaphyseal cross-pins [3,7,8]. It has been documented that use of a small diameter screw may predispose tendon slippage from the canal [4] and that with larger screws the graft can be damaged by the screw thread [1]. Other concerns regard the imposed anatomical interposition of the screw between one side of the bone canal and the transplant, which hinders direct healing there [9] and creates a non-physiological graft to bone transition.…”

“…However, due to poor performance of the pressure indicating film in the humid environment of actual bone, and observed experimental variation attributed to heterogeneity of the porcine bone density and structure [1,5], composite polyurethane foam Sawbone blocks were used as a test surrogate for human bone (Nr 1522-11, Sawbones Europe AB, Malmö, Sweden). The cancellous region of the bone had a density of 20 g/cm 3 [6] and was laminated with a 1 mm cortical layer of 64 g/cm 3 polyurethane foam.…”

“…For the case of hamstring autograft fixation, the screw forces the tendon graft into contact with the bone tunnel, creating pressure, friction and subsequent mechanical stability. Techniques in which an interpositional bone wedge is placed between the screw and the tendon have also been introduced, in an effort to decrease the risk of damage to the tendon and to create a more even and circumferential bone contact [1][2][3]. However, even though often used, the mechanical performance of both approaches might have the potential for optimization [3,4].…”