Only About Half of Reconstructed Knees Achieve Stability Symmetrical with the Other Knee

The goal of ACL surgery is to restore the preinjury level of stability, which should be the same as that of the other knee. True symmetry is achieved when the side-to-side difference (SSD) between the knees is 0. Measurement error increases this criterion to 1 mm. Thus we propose a SSD of 1 mm as defining knee stability symmetry. The IKDC “normal” criterion of up to a 2-mm difference may be satisfactory, but it is not truly normal. Indeed, a 2-mm SSD is what is commonly seen with partially torn ACLs.⁶⁷ When the 1-mm criterion is applied, we see the following: For all autografts, about 30% have greater than 2-mm SSD.⁶⁸ The remaining 70% fall into four categories: 2 mm, 1 mm, 0, or less than 0. If we assume that one-fourth of the 70% falls into each of these four categories, then it is reasonable to estimate that one-fourth of 70%, or 18%, are exactly 2 mm different. Adding this 18% (exactly 2 mm) to the 30% (greater than 2 mm) would mean that 48% of the reconstructed population has a 2-mm or greater SSD. This leaves about 52% with 1 mm or less SSD (i.e., true symmetry with the other knee). Thus roughly one-half of the autograft ACLRs, in the hands of the experienced knee surgeons who are the authors of these studies, have stability that is either equivalent to a partially torn ACL or worse. The allograft data⁶⁸ show significantly lower stability rates (see Table 69-1).

Table 69-1 presents the raw data for stability from all the studies. The principal areas of interest are the “normal” and “abnormal” stability columns. Abnormal stability in most cases is equivalent to graft failure. The primary table subdivision is by graft type. These are four-strand hamstring (4HS) autograft, two-strand hamstring (2HS) autograft, BPTB autograft, and quadriceps tendon autograft and allograft. The secondary subdivision is by graft subgroup and by fixation type. Subdividing by fixation groups is possible to do with the autografts because of the large number of studies. It is only possible with the allografts to break out a BPTB/interference subgroup because of the smaller number of studies.

Stability Rates by Graft Type

The principal findings were as follows:

Stability Rate by Graft Fixation Subgroups

Stability rates for these subgroups are as follows:

Aperture Versus Nonaperture Fixation

For soft tissue grafts, there was no stability advantage for aperture fixation. Indeed, as just described, the 4HS-EB2 group, which had entirely nonaperture cortical fixation, had the highest stability rates of any graft fixation subgroup. Thus the so-called “bungee effect” would appear to be nonexistent. This is not surprising, as studies have shown that fixation on rigid cortical bone enhances stiffness much more than the greater length of the construct diminishes it.⁶⁹ Also, variations in fixation stiffness are eliminated once the graft heals into the bone tunnel because the fixation is no longer load bearing. What really matters is how much the graft may elongate during this healing period from either slippage or plastic deformation. Neither of these is related to fixation stiffness.

Four-Strand Hamstring Versus Bone–Patellar Tendon–Bone Stability Rates

Because BPTB has long been considered the “gold standard” for ACLR. it may be surprising to some that 4HS was found to have higher stability rates. However, as the 4HS graft is a significantly stronger graft (see Chapter 10), the excellent 4HS stability rates do make sense if modern fixation is used. Prior analyses have seemed to show that hamstring grafts had lower stability rates than BPTB.^70,71 However, these studies commingled the much-lower-stability 2HS studies with the higher-stability 4HS studies. If the 2HS studies are removed, it turns out that there was no stability advantage to BPTB in those studies by comparison with 4HS. Also, many of the highest-stability 4HS series were published after those studies. They were thus not included in those analyses but contribute significantly to the higher 4HS stability rates found here.

Allograft Versus Autograft

Allograft use has been steadily increasing. Surgeons who wish to avoid the morbidity of BPTB as well as the harvest of hamstrings are among the many who have been turning increasingly to allografts. The data presented here show significantly lower stability rates for allografts, with an abnormal stability rate, which usually represents graft failure, about three times higher than for autografts. These low-stability results also would indicate that ligamentization may indeed proceed differently for allogeneic tissue. This may be due in part to the less-effective and slower rate of recellularization that occurs with allografts.⁷² There is anecdotal evidence that late failure rates may be higher for allografts,^73,74 which may be related to this same phenomenon.

Many now believe that some form of sterilization is necessary for allografts over and above sterile processing (see Chapter 70). Although higher levels of radiation have been believed to provide good sterilization, they have also long been considered to decrease graft strength. However, the radiated grafts in this study had lower stability rates than the nonirradiated grafts, even though the radiation levels in these studies were below 3 MRADs.^49,63 This level is below that required to kill the human immunodeficiency virus (HIV)⁷⁵ but is still apparently high enough to diminish graft strength. Some have felt the answer lies in radioprotectant that allows virucidal 5 MRAD level sterilization while potentially avoiding graft weakening.⁷⁶ Clinical studies to test this hypothesis are ongoing.

A variety of chemical treatments of allografts are also variably carried out by different tissue banks.^77,78 It is unknown how these treatments affect graft ligamentization, cellular repopulation, and ultimate strength. Careful clinical follow-up will be required to assess which allograft treatments are optimal regarding both sterility and stability.