The most important findings of the present study were that the diameter of the ALB graft and posterior slope angle of tibia were significantly associated with PTT after double-bundle PCL reconstruction with hamstring autografts. Namely, smaller ALB graft and flatted or shallowed tibial slope were considered as risk factors for postoperative PTT.
It is well known that the PCL functions as one of the main stabilizers of the knee joint and serves primarily to resist excessive posterior translation of the tibia relative to the femur 1. Fox et al. have reported that the ALB provided the primary restraint to posterior tibial translation when the knee is flexed to 90 degrees, and the PMB functions as the primary restraint to posterior tibial translation with the knee near full extension, as well as a secondary restraint to knee rotation 14. Race et al. reported that majority of the strength of the PCL comes from the ALB, since the tensile strength of the ALB is 1620 N, whereas the tensile strength of the PMB is 258 N 15. From these findings, the ALB is considered as the most important factor preventing posterior tibial re-translation after double-bundle PCL reconstruction at knee flexion of 90 degrees.
Previously, several options have been described for graft selection: an Achilles tendon with bone plug allograft, quadriceps tendon allograft, bone-patellar tendon-bone (BPTB) graft for larger ALBs, and a semitendinosus tendon autograft and allograft, tibialis anterior allograft, tibialis posterior allograft for smaller PMBs 4,16. Several surgical procedures and graft sizes were also introduced previously. Markolf et al. used an 11-mm BPTB graft for the ALB and an additional 8-mm BPTB graft for the PMB 17. Pache et al. preferred an 11-mm Achilles allograft for the ALB graft, and a tibialis anterior allograft 7 mm in diameter for PMB 2. Using allografts of sufficient size for ALB and PMB grafts might be better. However, in some countries, including Japan, allografts are not available for surgery. BPTB is one of the common grafts used for knee cruciate reconstruction. This type of graft has shown good stability, with bone-to-bone healing expected 18. On the other hand, the disadvantages of using BPTB were also reported previously, including donor site morbidity, reduced knee extension strength, anterior knee pain, and so on 19,20. In addition, Yoo et al. measured the geometry of the patellar tendon with knee magnetic resonance imaging, reporting that the mean patellar tendon width of male Korean adults (n = 142, height 175.7 ± 5 cm) had a proximal width of 30.3 mm and a distal width of 24.0 mm, and female adults (n = 30, height 162.6 ± 4.9 cm) had widths of 27.5 mm and 21.5 mm, respectively 21. Oikawa et al. also reported in a cadaveric study that the mean proximal, central, and distal patellar tendon widths were 29.9 mm, 27.3 mm, and 25 mm, respectively 22. Therefore, an over-8-mm BPTB autograft is larger than one-third of the patellar tendon width for Asian patients due to their physique. Milankov et al. reported patellar tendon rupture in 1.8% and patellar fracture after harvesting the BPTB in 0.45% 23. They recommended that, to minimize the risk of patellar fracture, no more than a 25- to 30-mm length of the patella and no more than 9 to 10 mm of its width should be removed 23. Moreover, Lin et al. concluded that a hamstring tendon autograft may be a better choice for transtibial tunnel PCL reconstruction compared with a patellar tendon autograft due to the lower incidence of anterior knee pain, squatting pain, kneeling pain, and osteoarthritic change 24. It is well known that the advantage of using soft tissue grafts such as the hamstrings include easy control of the size and length for graft preparation. For these above reasons, we use hamstring autografts for double-bundle PCL reconstruction.
However, as already mentioned above, the ALB is considered the most important factor preventing posterior laxity after double-bundle PCL reconstruction at knee flexion of 90 degrees. We usually harvested both a semitendinosus tendon graft and a gracilis tendon from the ipsilateral knee. Sometimes, a triple-folded or quad-folded semitendinosus tendon could not reach a sufficient diameter for an ALB graft in small physique people such as Asian patients. In these cases, a smaller diameter ALB graft may lead to postoperative PTT. When the harvested hamstring grafts were smaller (cutoff value was 6.5mm), harvesting an additional graft source to make a larger size graft or converting the surgical procedure from double-bundle PCL reconstruction to other procedures should be considered.
Recently, several authors have reported the relationship between posterior tibial slope angle and PCL injury. Schatka et al. reported that a high tibial slope was significantly correlated with increased posterior tibial translation 25. On the other hand, Bernhardson et al. reported that PCL graft forces increased as tibial slope decreased (flattened) when loaded, and they concluded that the effect of tibial slope on PCL grafts was the same as that which has been noted clinically, and a flat tibial slope should be considered a factor when evaluating the cause of failed PCL reconstructions26. Gwinner et al also reported that the flatted posterior tibial slope was associated with a significantly greater persistent PTT 27. In the present study, decreased posterior slope angle of tibia was one of the optimal risk factors for postoperative PTT. Furthermore, the posterior tibial slope angle of the failure group was more decreased compared with the good group with significance. Although, we did not perform the biomechanics investigation in this study, our results support the previous reports of Bernhardson and Gwinner.
Previously, preoperative grade 3 injury was the optimal risk factor associated with postoperative PTT has been reported 11. It was possible that preoperative grade 3 injury induces other soft tissue laxity such as articular capsule. However, there was no significantly difference of preoperative GSV between failure group and good group in the present study. In addition, there remains controversy concerning the graft fixation angle. Both AL and PM bundles were fixed at 90°of knee flexion according to Kimura’s report in this series 28. Tachibana et al had been reported that both grafts were fixed at 0° of knee flexion 11. On the other hand, Kennedy et al recommended that PM graft should be fixed at 0° and the AL graft should be fixed at 90°at knee flexion 29. These differences of graft fixation angle may influence postoperative PTT.
Several limitations must be taken into consideration with respect to the present study. First, the study had a small sample size and, second, it was not randomized. Third, the minimum follow-up period was 12 months after surgery. However, sequential change in PTT after surgery was confirmed in 3 months, and there was no significantly development between 3, 6, 12, and 24 moths has been reported 11. Fourth, the details of the mechanisms and timing of posterior laxity after surgery were obscure. However, despite these limitations, the present study may contribute to providing important information for double-bundle PCL reconstruction with a hamstring autograft.