We can attribute many of the approximately 200,000 annual anterior cruciate ligament injuries, and the resulting 100,000 surgical repairs, to high-level athletes who refuse to give up on their sport and weekend warriors who refuse to give up on the dream. As knees continue to twist and pop on high school football fields, college courts and neighborhood softball diamonds, advances in ACL reconstruction will remain a hot topic for outpatient facilities.

A developing double-bundle procedure is gaining momentum and may eventually become the preferred technique. But because the procedure is technically more difficult and results in longer case times than the traditional single-bundle ACL repair, the tried-and-true approach remains the gold standard. Here's an in-depth look at both techniques as you prepare for improvements in knee repair.

Room for improvement
Historically, primary repair of the ACL has been unreliable due to problems with blood supply in the repaired ligament. As a result, emphasis has shifted to reconstruction with various forms of grafts. The more common techniques have used portions of the patellar tendon or hamstring tendons as a substitute for the ACL. Unfortunately, outcome reviews have shown that a significant number of these procedures still have disappointing results. In particular, some athletes continue to complain of instability of the repaired knee, especially during cutting maneuvers.

Cutting movements bring in to play rotary instability, while deceleration involves only anterior tibial translation (forward gliding of the tibia on the femur). An anterior cruciate-deficient knee is prone to giving out during attempts to decelerate or cut. This often occurs in a non-contact situation and the problem is, therefore, not restricted to contact sport participants. In order to develop a procedure that would provide improved stability, the anatomy and biomechanics of the ACL have been reviewed with the aim of developing a more physiologic reconstruction.

The ACL limits anterior tibial translation and excessive rotation of the tibia on the femur (rotary instability). In order to accomplish these two functions, the ACL actually consists of two bundles: the anteromedial bundle and the posterolateral bundle. The AM bundle tightens when the knee flexes, the PL bundle tightens when the knee is in extension.

Biomechanical studies have isolated the functions of the two bundles in resisting forces on the knee at varying degrees of flexion. In resisting translation, the PL bundle was most effective when the knee was near extension, while the AM bundle was more effective at flexion angles greater than 30 degrees. The PL bundle was also found to be more effective in controlling rotation than the AM bundle.

Since the current single-bundle reconstruction technique involves drilling tunnels in the tibia and femur to place the new ligament more nearly in the position of the AM bundle, the biomechanical studies show that the knee's translation would be better controlled than its rotation. Indeed, complaints of rotary instability are responsible for most of the unsatisfactory ACL repair results.

Two for one?
The anatomic double-bundle technique of ACL reconstruction was developed in an attempt to more closely duplicate the biomechanics of the ACL, thereby improving knee function after reconstruction when compared to the traditional single-bundle technique. The double-bundle method uses two grafts to create a more anatomic replacement for the ACL. As with any new procedure, a variety of methods are currently being used. If this technique proves to be more effective, it can be expected that one or two variations will become standard.

With regard to graft selection, most of the standard tendon grafts have been employed. The trend is toward using hamstring tendon autografts, which have satisfactory loading characteristics without the theoretical graft site problems of the bone-patellar tendon-bone constructs. Some biomechanics experts are concerned that the use of hamstring autografts may result in weakened hamstring function. The hamstrings are known to be secondary stabilizers of the knee with respect to anterior translation. While this is a theoretical concern, it does raise the problem of robbing Peter to pay Paul.

Initially, the double-bundle technique was developed using two tunnels in the tibia and two in the femur. The advantage of limiting the number of bone tunnels both with respect to time of procedure and risk of technical complications has led to the development of alternative techniques. Surgeons can create one tunnel in the tibia and two in the femur or larger single tunnels in both locations. A standard procedure has not yet emerged.

Biomechanical studies on cadaver knees revealed that double-bundle reconstructions provided 93 percent of the strength of an intact ACL as compared to 68 percent for the single-bundle. Strength of the construct alone does not demonstrate superiority, however. If the new method is to have an advantage over the simpler single-bundle technique, it must be shown to be better at duplicating the functions of the ACL.

Laboratory comparisons of the ability of the two techniques to replicate the ACL's control of translation and rotary instability in the knee have demonstrated superiority of the double-bundle technique. While many studies have shown improved control of translation, the real advantage is the demonstrated ability of the double-bundle technique to better control rotary instability.

Biomechanical studies are useful, but the real test of a procedure is how it performs in real life. Although the technique is too new to have any long-term clinical follow-up results, some comparison studies have been performed. While initial clinical tests didn't demonstrate any significant difference in stability between the single and the double-bundle techniques, these studies looked primarily at translation.

When more sophisticated studies were performed to evaluate rotary instability, the double-bundle technique was found to be superior. Do the results translate to the repair's performance in the athletic setting, where the forces affecting the knee are greater and muscle function and coordination come into play? While both the biomechanical and early clinical evidence show that this new method of ACL reconstruction may indeed be an improvement over the traditional single-bundle technique, we need long-term follow-up studies of function under actual conditions before we can draw realistic conclusions.

In your facility
Surgeons and facility administrators must decide whether the double-bundle technique provides sufficient advantage over the traditional technique to justify the increased surgical time and more difficult surgical technique. It's unlikely that the double-bundle procedure will become the gold standard overnight. Surgeons tend to be a conservative group and are usually reluctant to jump on the bandwagon of a new procedure when they're comfortable with a technique that has been producing good results. More definitive proof of a significant advantage for the double-bundle repair must be shown before a widespread shift away from the traditional procedure occurs. This proof is at least two or more years down the road.

If surgeons do convert to the double-bundle technique in time, the transition should be thought of as an evolution of an existing procedure rather than a revolutionary shift in orthopedic care. It's unlikely that there would be a significant outlay for new equipment or a need for extensive training of OR personnel. As with any new procedure there will, of course, be a learning curve both for the OR staff and surgeon before you can achieve maximum efficiency. The core techniques won't change, but the sequence and number of tunnels to be drilled will.

But even when maximum efficiency is reached, the procedure will result in longer operating times than the traditional single-bundle technique. Estimates are that the double-bundle repair will add 15 minutes to 20 minutes per procedure. Even if surgeons can improve in their skills to add only fifteen minutes to each case, facility administrators need to prepare for an additional hour of OR use for every four procedures performed. Like it or not, economics is now a significant factor in medical decisionmaking and a new procedure will be expected to demonstrate clearly superior outcomes in order to justify this added room time.

With the current emphasis on early post-op rehabilitation following ACL reconstruction, the strength of the graft fixation is an important factor no matter which technique you use. Until the graft has healed in the bone tunnel, the fixation device - traditionally plastic or metal screws - must be able to resist the loads placed upon it by rehabilitation exercises. The ideal fixation device would provide the necessary resistance over the time required for healing without damaging the graft or allowing excessive graft motion within the tunnel.

Experimental work is currently being done with shape-memory polymers and alloys that may move us closer to an ideal solution. The materials would fit in the tunnel with the tendon graft, anchoring the tendon in a manner similar to an interference screw. In theory, this shape-memory anchor would maintain a constant fixation force on a graft despite changes in the bone tunnel's structure associated with the healing process. Human trials have not yet started and it is far too early to tell if this material will live up to expectations. It is, however, an exciting concept worth following.