Chapter 74 Tunnel Widening After Anterior Cruciate Ligament Reconstruction

Introduction

Tunnel widening after anterior cruciate ligament reconstruction (ACLR) has been noted for many years. Initially there were concerns that this widening would be progressive. However, it is now clear that tunnel widening only occurs in the first postoperative year ^1,² and is not progressive after that period. A number of variables have been hypothesized to cause or contribute to tunnel widening. These include the following:

1 Hamstring (HS) versus bone–patellar tendon–bone (BPTB) graft

2 Allograft versus autograft

3 Fixation location: cortical versus apertural

4 Rehabilitation: the aggressiveness of the postoperative rehabilitation protocol

5 Synovial fluid: the degree to which synovial fluid penetrates into the tunnel

These potential etiologies and the significance of tunnel widening will be discussed in this chapter.

Methods of Analyzing Tunnel Widening

Quantifying Tunnel Widening

There is no established standard method for quantifying tunnel widening. Most authors have compared tunnel widths at a specified point(s) within the tibial and/or femoral tunnels and calculated the increase in tunnel size relative to the original tunnel size. The elapsed interval of time after the surgery is also specified. Plain radiographs are usually measured, but computed tomography (CT) or magnetic resonance imaging (MRI) may also be used.

Literature Analysis

The literature with regard to tunnel widening can be evaluated in two ways. The first is to aggregate and analyze the amount of tunnel widening reported in all series that use one surgical technique and contrast it to that found in all series using a contrasting surgical technique (i.e., inter-series analysis). This can be used to compare, for example, mean tunnel widening in HS series versus BPTB series. The second method is to look at those series that use both techniques within a given series (i.e., intra-series analysis). Both methods were used in this analysis so that the entirety of the tunnel widening literature could be analyzed. Both methods contributed useful information. In general, good agreement was also found between each method for a given parameter.

Specific Factors Associated With Increased Tunnel Widening

Hamstring Versus Bone–Patellar Tendon–Bone Graft

It has been hypothesized that HS grafts are associated with more tunnel widening than BPTB grafts. Tunnel widening has been found with the use of both grafts. Seven series specifically compared HS with BPTB. However, because fixation is different for the HS and BPTB grafts in all these series and because fixation type is one of the leading hypothetical causes of tunnel widening, the isolated effect of graft type is difficult to discern. Of these seven series,²^–⁸ two report tunnel widening that is roughly equal for HS and BPTB and five report more tunnel widening with HS. These seven series are presented in bar graph form in Fig. 74-1.

Fig. 74-1 Tunnel widening by graft and fixation type. BPTB, Bone–patellar tendon–bone; HS, hamstring.

Two clinical series ^9,¹⁰ have shown increased tunnel widening in allografts versus autografts. In addition, a recent sheep study by Weiler showed increased tunnel widening in allografts at all time periods measured, beginning at 6 weeks and ending at 1 year.¹¹ Possible causes include an increased immunologic response to the graft or the presence of chemical residua in the grafts from processing, cleansing, cryoprotectant, radioprotectant, or the like.