Bone tunnel enlargement after anterior cruciate ligament reconstruction: fact or fiction?

Radiographic enlargement of bone tunnels following anterior cruciate ligament (ACL) reconstruction has been recently introduced in the literature; however, the etiology and clinical relevance of this phenomenon remain unclear. While early reports suggested that bone tunnel enlargement is mainly the result of an immune response to allograft tissue, more recent studies imply that other biological as well as mechanical factors play a more important role. Biological factors associated with tunnel enlargement include foreign-body immune response (against allografts), non-specific inflammatory response (as in osteolysis around total joint implants), cell necrosis due to toxic products in the tunnel (ethylene oxide, metal), and heat necrosis as a response to drilling (natural course). Mechanical factors contributing to tunnel enlargement include stress deprivation of bone within the tunnel wall, graft-tunnel motion, improper tunnel placement, and aggressive rehabilitation. Graft-tunnel motion refers to longitudinal and transverse motion of the graft within the bone tunnel and can occur with various graft types and fixation techniques. Aggressive rehabilitation programmes may contribute to tunnel enlargement as the graft-bone interface is subjected to early stress before biological incorporation is complete. Further basic research is required to verify the effect of the various proposed factors on the etiology of bone tunnel enlargement. We recommend that routine follow-up examinations after ACL reconstruction should include the measurement of bone tunnel size in order to contribute to a better understanding of the incidence, time course, and clinical relevance of this phenomenon. Improved and more anatomical surgical fixation techniques may be useful for the prevention of bone tunnel enlargement.