Increasing posterior tibial slope does not raise anterior cruciate ligament strain but decreases tibial rotation ability
Introduction
High tibial osteotomy has been performed as a treatment for knee osteoarthritis to correct varus deformity for many years. The long term success of the traditionally performed closed-wedge osteotomy has been proven by different authors (Akizuki et al., 2008, Amendola and Bonasia, 2010, Benzakour et al., 2010, Koshino, 2010). As major advantages of the open-wedge osteotomy compared to the closed-wedge osteotomy have been described medial open-wedge osteotomy has become the treatment of choice to correct varus deformity. The open-wedge technique avoids fibular osteotomy with the risk of malunion, dissection of the peroneal nerve and detachment of the tibialis anterior muscle (Lobenhoffer et al., 2004, Luites et al., 2009, Song et al., 2010).
Additionally, open-wedge osteotomy allows the correction of knee deformity in the sagittal plane as well. On lateral radiographs the mean tibial slope of the knee is 10° (SD 3°) (Dejour and Bonnin, 1994, Genin et al., 1993). It has been postulated by different authors that the posterior tibial slope (PTS) is a major factor affecting anterior–posterior knee stability (Dejour et al., 1994, Giffin et al., 2004, Giffin et al., 2007, Hernigou et al., 1987, Kostogiannis et al., 2011, Rodner et al., 2006, Terauchi et al., 2011, Todd et al., 2010). This is supported by radiographic studies that showed a linear relationship between PTS and tibial translation (Bonnin and Chambat, 2004, Dejour and Bonnin, 1994). Liu and Maitland found a relation between anterior tibial translation and posterior tibial slope in anterior cruciate ligament (ACL) deficient knees. Anterior tibial displacement increased from 9.1 mm for a tibial slope angle of 4° to 15.2 mm for a tibial slope angle of 12° (Liu and Maitland, 2003).
Several biomechanical studies have shown that increasing the posterior slope of the tibial plateau shifts the resting position of the tibia anteriorly and therefore causes a change in knee kinematics and thus affecting the distribution of contact pressure (Agneskirchner et al., 2004, Fening et al., 2008, Giffin et al., 2004). Rodner et al. described similar findings in ACL deficient knee joints (Rodner et al., 2006).
Giffin et al. found an anterior shift in the resting position, which was accentuated under axial loads, but no increased translation under anterior tibial shear loads after increasing the tibial slope (Giffin et al., 2004). From these results they concluded that decreasing slope may be protective in an ACL-deficient knee.
Brandon et al. found in a radiographic study an association between increased PTS and anterior translation and tensioning of the ACL. This may cause ACL laxity and rupture (Brandon et al., 2006).
In the literature there is conflicting evidence whether an increased tibial slope is a risk factor for noncontact injury of the ACL (Todd et al., 2010). Whereas the results of the study of McLean et al. supported the concept that the PTS is an important risk factor, Meister et al. did not find an increased slope to be a risk factor for noncontact injury of the ACL (McLean et al., 2011, Meister et al., 1998).
As a higher PTS can potentially result in an anterior shift of the tibia this can possibly influence in situ forces on the anterior cruciate ligament. Different authors have therefore recommended operative correction of the PTS to improve joint stability (Agneskirchner et al., 2004, Giffin et al., 2004, Giffin and Shannon, 2007, Imhoff et al., 2004). But the relation between tibial slope and ACL strain and its influence on joint stability is still poorly understood.
Although there are several studies evaluating the effect of PTS on sagittal stability, the role of the PTS on rotational stability has not been studied to the same extent. A positive pivot shift is clinically used to assess rotational instability, Brandon et al. and Voos et al. showed an association between increased PTS and higher pivot shift grades (Brandon et al., 2006, Voos et al., 2012).
The hypothesis of this biomechanical in vitro study was that increasing tibial slope would result in increased strain of the ACL and hence increased risk of failure. We additionally hypothesized that increasing tibial slope would influence knee kinematics.
Section snippets
Methods
Posterior slope angles of 5°, 10° and 15° were created in 9 human cadaveric knee joints (South East Tissue Alliance, Gainesville, Florida) by osteotomizing the tibiae proximally of the tuberositas tibiae and fixing the osteotomy in the respective slope angle with an external fixator. Based on results of a study on the influence of tibial slope on anterior tibial translation (Dejour et al., 1994) an a-priori power analysis (d = 3.3, α = 0.05; 1-β = 0.8; ∂ = 4.04; t = 2.78) was performed to calculate the
Results
The mean ACL strain amplitude defined as difference between maximally and minimally occurring strain during flexion–extension cycles, ranged between 2.8 and 6.1% depending on the load case (Fig. 4). Increasing the posterior tibial slope up to 15° did not lead to statistically significant alteration of ACL strain in most of the load case. However, the ACL strain amplitude significantly decreased under an external rotation moment, when stepwise increasing the slope angle from 0° to 15° (Fig. 4).
Discussion
In contrast to the first postulated hypothesis of this study, increasing the posterior tibial slope by up to 15° did not increase anterior cruciate ligament strain under none of the tested loading conditions. Increasing the posterior tibial slope, however, affected knee kinematics in terms of a reduced extent of tibial rotation during flexion–extension cycles in most of the load cases tested, which confirmed the second hypothesis of this study.
Giffin et al. examined knee kinematics in ten
Conclusion
The results of the present study suggest that an increased posterior tibial slope does not result in increased strain of the ACL. Therefore, from a biomechanical point of view, osteotomies performed to correct the tibial slope as carried out by some clinicians to stabilize ACL deficient knees might not be effective. In addition, increasing tibial slope can reduce the extent of tibial rotation up to 5° during a knee joint flexion exercise, which may not be desirable and at this time is of
Acknowledgement
We kindly acknowledge the assistance of Patrizia Horny in preparing the illustrations.
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