Purpose: To compare the radiographic tibial tunnel enlargement at one year post operatively following anterior cruciate ligament reconstruction in two patient groups one operated on soon after an acute injury and the other for chronic instability. Also, to correlate the radiographic findings with the clinical results.

Patients and Methods: The study group was of 42 patients who underwent isolated central anterior cruciate ligament reconstruction with a four thickness hamstring tendon technique. 20 of these were consecutive patients with a mean injury to operation delay of 4.5 weeks. Twenty two were consecutive patients with chronic instability and a mean injury to reconstruction interval of 29 months. All patients were reviewed at year post-operatively, their assessment including clinical examination, Lachman and Jerk Tests, arthrometer testing, Tegner activity level recording and weight bearing AP and lateral radiographs.

Tibial tunnel diameters were measured by two independent observers at two points. The proximal measurement was made 5 mm from the tibial articular surface and the distal, 5 mm from the lower end of the tunnel. The tunnel enlargement was calculated from the known drill size after correction for magnification.

Tunnel enlargement was compared between the two groups, was correlated with the clinical findings and the results were analysed statistically.

Results: Tibial tunnel enlargement was seen in both groups (p< .001). The enlargement was significantly greater at the proximal end of the tunnel (34%) than at the distal end (25%) (p< .05). In the acute group the mean increase in the tunnel diameter at the proximal end was 31%. In the chronic group it was 36%. This difference however was not statistically significant. (P> .05). At the distal end the mean tunnel enlargement in acute and chronic groups was 24% and 27% respectively. (P> .05).

Tunnel enlargement was significantly higher in patients with persistent effusion at one year. (40%:31%) p< .05. We did not find any correlation between tunnel enlargement and clinical outcome.

Conclusion: Anterior cruciate reconstruction by an isolated central hamstring tendon technique, carried out sub-acutely following injury, does not significantly reduce the incidence of tibial tunnel enlargement as compared with knees operated on by the same technique for chronic instability. There was a significant association between tunnel enlargement and the persistence of effusion.