Alternative ACL Fixation Device

For my Surgery for Engineer's project, my team did an evaluation of the current techniques utilized for Anterior Cruciate Ligament (ACL) repair. After reviewing the fixation techniques, we determined that current ACL fixation devices lack the ability to tension a graft post-fixation. Therefore, we developed TensiFix -a device that utilizes an interference screw to provide post-fixation tensioning.

Intro

A graft must be fixed to return the knee to an optimal stability. Although it would seem that graft strength is the most indicative property for ACL reconstruction stability, it is actually the strength of the graft fixation to the bone. (Urbibe, Arango, Frank, and Kiebzak, 2013) Fixation gives the graft bone tunnel interface necessary stiffness and strength (Woo, et al. 2000). There are many different fixation devices available, such as interference screws, soft tissue washers, suture-post constructs, simple staples, cross-pins, and titanium buttons (Woo, et al., 2000). Since the graft selection depends on the circumstance of the patient, different fixation techniques must be used as well. The current gold standard for fixation is the interference screw against a bone plug (Brand, 2000). One study suggests the most stable knee reconstruction is when the interference screw is positioned close to the articular surface with central and distal fixation (Ishibashi, et al., 1997). Furthermore, the tibia position relative to the femur when the graft is fixed has a considerable effect on the kinematics of the knee and the in situ force of the new graft (Hoher, et al., 1999). Specifically, when the graft is fixed at 30o knee flexion, with a 67N posterior tibial load, the resulting knee kinematics is closest to the native knee (Hoher, et al., 1999).

Graft Tension

Joint kinematics and in situ forces of the graft while the knee moves are considerably altered by the initial tension of the graft (Woo, et al., 2000). If the initial graft tension is low, there necessary joint stability will not be provided (Woo, et al., 2000). Conversely, high initial graft tension would hinder joint motion (Woo, et al., 2000). The initial tension is suggested near 44N but this is controversial since the native ACL force is unknown (Woo, et al., 2000)

Problem

Currently, surgeons must rely on their own strength to add tension to the ACL graft prior to fixing the graft. Once the graft is fixed, there is no way to add more tension if it is needed. Since surgeons must individually tension each graft, there is a lot of variance between graft tensions across all ACL reconstruction patients. Furthermore, over time, the graft naturally loses tension with patient usage. There is no method to re-tension a graft other than performing a new ACL surgery.

Our Device

The TensiFixTM fixation device allows the surgeon to fix a graft at both ends, and then add more tension on the femoral side. When the graft loosens over time, a patient can choose a minor surgery where the surgeon goes back to the original graft and tighten TensiFixTM, thus adding more tensions without performing another major surgery.

Innovation

Post surgically, ACL grafts have been seen to lose tension due to normal cyclic loads applied during rehabilitation (Singhal, et.al, 2005). As the tension in the graft decreases, the knee’s ability to translate anteriorly increases; an occurrence meant to be minimized by the ACL reconstructive surgery (Grover, et. al, 2005). With current fixation techniques, the amount of tension in the graft immediately after surgery is dependent on that which the surgeon can provide in the tibial fixation. With our device, the surgeon is able to add additional tension to the graft at the femoral site after both ends of the graft are fixed.

Impact

TensiFixTM’s surgical importance lies in its ability to allow the adjustment of the graft after it has already been fixed in place. Currently, ACL reconstruction grafts are tensioned by manually pulling on the graft before fixation (Fu, Cohen 2006). This method can have many pitfalls. For example, variability in graft tension is associated with manual tensioning, as manual methods have been proven to be irreproducible from patient to patient (O’Neill, et al 2011). Also, fixation after tensioning limits the surgeon’s inability to further tension the graft if need arises. Thus, if the graft were to either not have enough tension initially or lose tension over time due to factors such as lengthening due to cyclic loading, the surgeon would have to redo the reconstruction to add tension to the graft (Harvey, et al. 2005). TensiFixTM functions like a regular Endobutton, allowing the surgeon to manually tension and fix the graft using the current manual method. However, should the fixed graft somehow lose tension, TensiFixTM’s tensioning screw can be backed out of its barrel, re-tensioning the graft. Thus, the surgeon is not forced to redo the entire reconstruction to tension the graft.

Significance

TensiFixTM is able to easily increase the tension during the ACL reconstruction. It addresses the need for a reliable way to increase the tension in knee ligament reconstruction surgeries. It uses an adjustable barrel-screw mechanism that is capable of adding tension to an existing graft. The TensiFixTM consists of three components: an externally threaded suture retainer, an internally threaded hexagonal barrel and a flat button retainer.

The device comes pre-assembled before being implanted. The surgeon will thread a suture through the hexagonal hole in the flat button and though the externally and internally threaded barrel. The suture is then attached through the hole for each component. The barrel will come pre-assembled in the lowest position possible to allow for the greatest range of tension to be applied. The suture is then looped through a double bundle graft. The device with the graft attached to it will be pulled through the end of the femoral tunnel and mounted at the end of the tunnel. The surgeon then places the barrel/retainer assembly into the hexagonal button and the graft is threaded through a femoral tunnel similar to the currently used EndoButton procedure. The TensiFixTM is then screwed out. It has the added benefit of being able to be adjusted post operatively. This allows surgeons to increase the graft tension in a quick and measurable way.