Introduction

Graft strength is only one of the factors influencing anterior cruciate ligament (ACL) graft choice. However, it has a direct bearing on ultimate stability, which is the goal of ACL. The relative strengths of potential ACL grafts are often not clearly appreciated. It is the purpose of this chapter to present the available data on the relative strengths of tendons that can be used as ACL reconstructive grafts.

Methods

Table 10–1 summarizes the data that we were able to find in the literature on graft strengths. Load to failure (LTF) is the parameter compared in each case. This data was found from computerized literature searches targeting ACL reconstruction and each of the specific grafts in clinical use. Although some tissue banks have performed their own studies on graft strengths, we have purposely excluded such proprietary data and relied only on data published in the peer-reviewed literature to avoid bias.

Comparison of Graft Strengths

There is significant variability in LTF results among different studies for the same graft (see Table 10–1). This is likely related to differences in testing methodologies. Thus, it is necessary to look at the totality of the data to get an overall idea of relative graft strengths. Some of the authors have what appears to be outlier LTFs, but the relative strengths between grafts within their study generally reflect the bulk of the literature. The two main examples here are the data of Brahmabhatt²¹ and Harris, with low LTFs for all grafts tested relative to other studies. When comparing grafts it is important also to notice the configuration of the tested grafts; in other words, whether it is a single, double, or quadruple graft, and in the case of bone–patellar tendon–bone (BPTB), whether it is a 10-mm or 15-mm graft. The data in Table 10–1 also show that braiding of grafts has been shown to weaken rather than strengthen grafts and is not clinically indicated.

Effect of Ligamentization

Chapter 55 describes the effects of ligamentization on graft strength. Although there is some disagreement, it appears that grafts retain only about half their initial strength at long-term follow-up. Thus, grafts that are significantly stronger than the native ACL at time zero may indeed be necessary to produce ultimate strengths that are as strong as the ACL initially was. Indeed, some studies report a lower re-rupture rate for reconstructed ACLs than for the contralateral normal ACL,¹ perhaps due to greater graft strength.

Allograft Strengths

Autograft strengths are relatively straightforward to measure. However, allograft strengths are more complicated because of the varying effects of graft preparation and sterilization techniques on the graft. Thus, any study that measures allograft strength can only be considered accurate for an allograft prepared in a similar manner. The relevant parameters may include radiated versus not radiated, the amount of radiation, and whether or not a radioprotectant was used. Other processes shown to significantly affect tissue properties include the use of cryoprotectant² and even simple freezing.³ This is further complicated by the fact that these parameters may affect allograft strength at longer-term follow-up by influencing revascularization and cellular repopulation in addition to their effects at time zero. Thus, time zero data may not be sufficient for comparison between autografts and allografts, particularly in light of evidence that late failure rates may be higher for allografts than for autografts.^4–7 The literature has also shown overall lower stability rates for allograft BPTB versus autograft BPTB,^8–17 suggesting that ligamentization may weaken allografts more than autografts.¹⁸

Quadriceps Tendon Graft Strength

The only published data we could find was from Brahmabhatt and Harris. Their LTF for the quadriceps tendon (QT) graft is quite low. However, we believe the proper way to interpret these data is by comparing them with their BPTB data, which are also much lower than other studies, probably due to testing methodology. The important point is that the QT LTF values in both of these studies are each about 20% stronger than the LTF for 10-mm BPTB tested the same way. Therefore, it would appear that a QT graft is likely a little stronger than a 10-mm BPTB graft.

Relative Strength of Hamstring and Bone–patellar tendon–bone Grafts

The classic paper of Noyes et al¹⁹ first compared various tissues with the ACL from the same cadaveric specimen. Other works have followed a similar methodology. These studies are summarized in Table 10–1. It should be noted that the study by Noyes et al used a 15-mm BPTB graft, whereas in practice a roughly 10-mm graft is used. Extrapolating from their numbers, a 10-mm BPTB graft would be 110% as strong as the native ACL. A two-strand semitendinosus (ST) and two-strand gracilis (Gr) (2ST/2Gr) graft would be 238% as strong as the native ACL. A four-strand ST (4ST) would be 280% as strong as the ACL. The real values for these multistrand grafts are probably a little less than these extrapolations because it is unlikely that the entire tendon is as strong as these index values. Some more recent studies have produced very different absolute numbers, perhaps related to testing methodological differences.^20–29 However, within studies the relative strengths of various grafts show general agreement.

Overall Relative Graft Strengths

Overall, 4ST and 2ST/2Gr grafts would appear to be the strongest available grafts in common use. Two-strand tibialis grafts are nearly as strong, followed by 10-mm BPTB and peroneus grafts, which are roughly two-thirds as strong as four-strand hamstring grafts. It should be pointed out that 15-mm BPTB can be used as an allograft and will more closely approximate the strength of a four-strand hamstring graft. The only data of which we are aware on tendo Achilles grafts show a low LTF. From the high girth of the graft it is likely, however, that a full-thickness tendo Achilles graft has much greater strength. This supposition will require more testing for validation.

Conclusions