Considerations of the Posterior Tibial Slope in Anterior Cruciate Ligament Reconstruction: a Scoping Review

Abstract

Purpose of Review

The significance of posterior tibial slope (PTS) in the setting of anterior cruciate ligament (ACL) injury and reconstruction has been increasingly recognized in recent years. The purpose of this article is to review the biomechanical and clinical studies of PTS in conjunction with ACL injuries, providing an evidence-based approach for the evaluation and management of this patient population.

Recent Findings

Several biomechanical and clinical studies suggest that PTS > 12° may be considered with increased strain on the native ACL fibers (or reconstructed graft) and greater anterior tibial translation, predisposing patients to a recurrent ACL injury. The increased rates of ACL injury and graft failure seen in those with increased PTS have garnered attention to diagnose and surgically address increased PTS in the revision ACL setting; however, the role of a slope-reducing high tibial osteotomy (HTO) in primary ACL reconstruction (ACL-R) has yet to be defined. Various HTO techniques to decrease PTS during revision ACL-R have demonstrated promising outcomes, though conclusions are limited by the multifactorial nature of revision surgery and concomitant procedures performed.

Summary

Recent evidence suggests that increased PTS is a risk factor for failure following ACL-R, which may be mitigated by a slope-reducing HTO. Further investigation is needed to elucidate abnormal PTS values and to determine appropriate indications for a slope-reducing HTO in primary ACL-R.

Introduction

Although optimizing surgical technique is an essential aspect of minimizing failure rates and poor outcomes following anterior cruciate ligament reconstruction (ACL-R), understanding the reasons for failure is at least of equal importance. The reasons for failure of ACL-R are usually multifactorial, including both technical factors as well as anatomical features and variations. Some of the most common technical factors associated with graft failure are reported to be femoral tunnel malposition, allograft use in younger patients, as well as thinner graft diameter [1, 2]. Additionally, some patient-related risk factors including hyperlaxity, young age, return to contact pivoting sport, meniscal deficiency, and preoperative high-grade rotatory laxity have also been associated with graft failure following ACL-R [2, 3].

Growing interest has been recently developed in patient-specific bony morphologies in pre-operative planning for ACLR. Thus, anatomically variable measurements have been considered risk factors for both anterior cruciate ligament (ACL) injury as well as for ACL-R failure, including narrow intercondylar notch, deep lateral femoral notch, posterior tibial slope (PTS), and varus malalignment [2, 4]. Among these anatomic considerations, the PTS has garnered special attention due to its propensity to cause anterior tibial translation (ATT) during weight-bearing activities, placing greater strain on the ACL. However, biomechanical and clinical investigations examining the role of PTS on ACL graft failure are still relatively new and thus, controversy still persists about its relative importance as an anatomic risk factor. Therefore, this review purposes to both describe the background of the PTS, the biomechanical principles influencing its effect on the ACL, as well as the clinical sequalae as they relate to graft failure. Additionally, this review will provide expert level recommendations regarding evaluation of the PTS as a risk factor for ACL-R failure aiming to answer how to account for it during preoperative planning and ACL-R.

History of the Tibial Slope

PTS has been increasingly recognized as a risk factor for ACL revision and an important consideration in the treatment regimen for ACL injury [5]. Additionally, the increased use of cross-sectional imaging modalities, such as computed tomography (CT) and magnetic resonance imaging (MRI), has provided an alternative means for surgeons to measure PTS.

Measuring the PTS

An accurate and reproducible measurement of PTS is necessary for clinical decision-making, as well as preoperative planning for osteotomy procedures. While several different methods are described in the literature based on advanced imaging modalities [6], a standard lateral knee radiograph is commonly utilized for the “circle three-point method” (Fig. 1) [7•]. The PTS is measured as the angle between the longitudinal axis of the tibia and the posterior inclination of the tibial plateau. A PTS > 12° has been suggested to be a risk factor of ACL-R failure [8]. As a result, a PTS > 12° has become an indication to consider the utility of increased tibial slope correction in ACL-deficient knees, especially in the revision setting.

Fig. 1.

Lateral radiograph of the knee with “circle three-point method” [7•] to illustrate measurement of the posterior tibial slope. PTS—posterior tibial slope

Biomechanical Effects

Current understanding of biomechanics for increased TS originated from studies demonstrating the effects of slope-producing and slope-reducing high tibial osteotomies on ATT in relation to the femur [9, 10]. Increased ATT due to increased PTS resulted in an anterior shift in tibiofemoral contact area and pressure [9], which was further magnified by an axial compressive load seen with weightbearing [10]. The effect of increased PTS on anterior tibial translation with externally applied forces was expanded upon in a cadaveric study assessing the effect of PTS in ACL-intact knees, suggesting that greater PTS was a risk factor for increased ATT during applied loads of axial, anteriorly directed, and valgus forces [11]. Additionally, increased PTS has been shown to alter normal knee kinematics even with activities of daily living, as demonstrated in a recent computer modeling study that found an association between pathologic PTS with both ATT and increased ACL forces during simulated activities such as walking, squatting, and standing [12].

There is evidence for the role of PTS in ATT and subsequent knee stability in both ACL intact and deficient knees from biomechanical studies assessing the role of PTS alteration. In a series of three cadaveric studies, the effect of PTS was assessed in ACL-deficient knees. In each of these studies, increased tibial slope was associated with increased ATT, whereas slope reduction mitigated this effect [10, 13, 14•]. Another study found that isolated slope-reducing osteotomy, as well as combined slope-reducing and varus-correcting osteotomy, results in diminished ATT, regardless of the integrity of the ACL [15•].

With respect to graft forces in ACL-reconstructed cadaveric knees, one study demonstrated that increasing tibial slope between a range of −2° to 20° under axial load resulted in an increase in ACL graft forces [16••]. Furthermore, another study demonstrated that a 10° slope-reducing osteotomy resulted in reduced ACL-R graft force in axially loaded knees with increased PTS [11, 14•]. Attempts have been made to quantify this exact graft force reduction, with one study concluding that a combination of slope-reducing and varus-correcting osteotomies reduced graft force by 33% and 58% under 200N and 400N compressive axial forces, respectively [15•]. While increased forces may be due in part to the increased ATT, other studies have also found that graft impingement as a result of increased PTS is another etiology of increased ACL force, especially with internal rotation [11]. These observations of increased graft forces in the setting of increased PTS illustrate the consequences of abnormal PTS on knee stability. Extrapolation of these findings suggests that, when treating ACL rupture, consideration should be given to abnormal PTS to minimize risk of re-injury.

Clinical Effects

The clinical impact of abnormal PTS is consistent with the findings from biomechanical studies. An initial radiographic study quantifying anteroposterior tibiofemoral laxity showed a 10° rise in PTS correlated with a 3-mm increase in the Lachman test in both ipsilateral and contralateral knees for patients with ACL tears [5]. Additionally, an early retrospective clinical study demonstrated the association of an increased PTS with ACL injury, as well as a concomitant high-grade pivot shift [17]. Moreover, the magnitude of ATT has demonstrated a positive correlation with PTS in both ACL-intact and ACL-injured patients [18].

Some clinical studies showed an increased risk for graft failure following ACL-R in patients with abnormal PTS alignment [3, 19–23]. A study evaluating adolescent and adult patients undergoing hamstring tendon autograft ACL-R identified PTS to be the strongest predictor of ACL reinjury, which was maintained at 20-year follow-up [19]. Furthermore, it was demonstrated that a markedly higher risk of graft failure existed in patients with a PTS > 12° compared to those with PTS < 12°, which was further pronounced (resulting in an 11-fold risk) in adolescents. This clinical impact of PTS was further solidified by recent studies, reporting the relationship between increased PTS and recurrent ACL injuries [8, 20, 22, 24]. While a PTS >12° predisposes the ACL graft to excessive stresses and subsequent failure, there is evidence that a pathologic PTS may also contribute to ACL reinjury by facilitating graft roof impingement after ACL-R [25], as previously postulated in biomechanical studies. While the clinical relevance of the more localized lateral (LPTS) and medial posterior tibial slopes (MPTS) are beyond the scope of this current discussion, a recent systematic review demonstrated an independent association of both increased LPTS and MPTS with ACL-R graft failure [4]. In conclusion, the current clinical evidence suggests that addressing a PTS > 12° may improve outcomes in revision ACL-R.

Treatment

Reduction of PTS is a technically challenging procedure that is a culmination of understanding of the biomechanical and clinical implications of an abnormal PTS. The goal of slope reduction osteotomy is to protect the ACL (or newly reconstructed ACL) by minimizing forces and strain. Currently, there is a lack of clear evidence for the ideal slope reduction, and further research is required to improve evidence of this surgical procedure. Thus, it is important to perform a thorough clinical and radiographic assessment of the patient’s knee malalignment, consider the need for any concomitant soft tissue procedures, and understand the effects of bony correction in multiple planes when selecting an osteotomy procedure. This section highlights indications and contraindications of the three most common slope reduction techniques, as well as the common concomitant procedures and clinical outcomes.

Anterior Closing Wedge HTO

One slope reduction HTO technique is an anterior closing wedge HTO (ACW-HTO), also called the tibial deflexion osteotomy. The ACW-HTO may be indicated in patients with previous ACL-R who have recurrent instability with PTS greater than 12° [26]. Although this procedure is seen most frequently in the revision and re-revision setting, there is a growing interest in expanding the indication to the primary setting in younger patients with more markedly increased PTS (>16–20 degrees). Thus, the current evidence on this clinical scenario is limited. Additionally, any varus deformity greater than 5° is considered as contraindication for this form of osteotomy [26, 27]. Another important contraindication for ACW-HTO is severe (grade IV) tibiofemoral osteoarthritis, as well as baseline knee hyperextension greater than 10° [26–28].

Currently, there are two main methods of performing the ACW-HTO, which differ based on the management of the tibial tubercle. In one technique, the osteotomy cut goes through the tibial tubercle [29]; whereas the other technique utilizes an osteotomy proximal to the tibial tubercle to prevent violation of the tubercle and the extensor mechanism [30]. Figure 2 illustrates the differences in these osteotomy techniques.

Fig. 2.

Two techniques of anterior closing wedge high tibial osteotomy, superimposed on a lateral radiograph of the knee. Osteotomy is made (A) at the level of the tibial tubercle [29] or (B) proximal to the tibial tubercle [30]. Note the osteotomy sites indicated by the dashed lines, illustrating maintaining distance from the posterior tibia to prevent hinge fracture. Yellow circle—tibial tubercle

The osteotomy made in the ACW-HTO is biomechanically advantageous in both knee flexion and extension. Additionally, by using a direct anterior approach, it is possible to visualize previous ACL tibial tunnels, allowing for simultaneous bone grafting [27]. Another advantage of the anterior approach in this technique is that there is little risk of PCL injury. Lastly, by making cuts to the anterior tibia, the surgeon can manipulate the tibial tubercle, specifically distalizing it to avoid altering patellar height [27].

Although the ACW-HTO implies many benefits and great treatment opportunities, there are some existing disadvantages, including risk of treatment-related complications. For instance, the utilization of an anterior cut and the anterior-posterior drilling have been earlier reported to increase both the risk of resultant genu recurvatum [27] as well as damage to the popliteal neurovascular structures [28].

Medial Opening Wedge HTO

In the case of increased tibial slope that is accompanied by varus malalignment, different slope reduction osteotomies may be more useful including the medial opening wedge HTO (MOW-HTO). Generally, the MOW-HTO is indicated in patients with genu varum and isolated medial compartment arthritis [31, 32], and it has been previously described to be effective in correcting coronal malalignment as well as contribute to alteration of the tibial sagittal balance, particularly the PTS. Even if MOW-HTO is traditionally thought to increase PTS [33], modifications to the surgical technique have been made to both maintain PTS [34] and also decrease the tibial slope in the sagittal plane [35•]. These modifications and subsequent changes to the PTS have been further related to the hinge axis position [36]. Specifically in the case of PTS reduction, intraoperative recommendations include maximal internal rotation and proximalization-extension of the hinge axis, creating an anterolateral hinge axis that results in slope reduction [37••].

There are many advantages to MOW-HTO. Mainly, by making the osteotomy on the medial side, the tibial bone stock is preserved, the fibula is spared, and the common peroneal nerve is avoided [35•]. Another advantage is that with MOW-HTO, intra-operative adjustments to the osteotomy angle can be made, increasing the precision of the osteotomy [37••]. Additionally, MOW-HTO can be combined with concomitant ligamentous procedures, increasing the potential for maximal restoration of native knee stability [35•]. Lastly, although in general HTO makes subsequent primary total knee arthroplasty (TKA) more complex [38], doing so after MOW-HTO may be more advantageous due to the increased technical challenge resulting from removal of the proximal tibial bone stock [39, 40], as well as the risk of lateral displacement of the tibial mechanical axis and tibial insert impingement on the endosteal cortex seen with a LCW-HTO [41]. In contrast to the ACW and LCW techniques, a disadvantage specific to the MOW-HTO is the increased risk of nonunion due to decreased bony apposition and compression. These risks should be considered in patients with a decreased capacity for bone healing, such as smokers and diabetics.

Lateral Closing Wedge HTO

The third option for slope reduction HTO is the lateral closing wedge HTO (LCW-HTO). Similar to the MOW-HTO, LCW-HTO is often indicated for coronal malalignment, chiefly isolated medial compartment arthrosis. Contraindications are similar to those previously mentioned, including older patients and grade IV tibiofemoral osteoarthritis. However, a prior lateral meniscectomy has been reported as a relative contraindication specific to LCW-HTO [42].

The LCW-HTO is advantageous, especially relative to MOW-HTO, due to its natural tendency to decrease the PTS [42, 43], providing multiaxial stability that is not seen with the ACW-HTO or traditional MOW-HTO. However, the LCW-HTO does have some disadvantages, including an increased risk of damage to the peroneal nerve with the laterally based osteotomy [44]. Additionally, intraoperative adjustment of the osteotomy angle is less feasible while performing the LCW-HTO leading to variability of correction and overall decreased precision [42]. Unlike the MOW-HTO, the LCW-HTO results in metadiaphyseal bone loss and changes in tibial condylar offset [45]. Additionally, post-surgical complications are higher for LCW-HTO. Specifically, in one study looking at more than 400 patients undergoing LCW-HTO, there was a nearly 50% rate of hardware removal [46]. Given these disadvantages and possible risk of under- or over-correction, it is important to consider alternative options, especially when the primary goal is to correct the tibia in the sagittal plane. Table 1 summarizes the advantages and disadvantages of the three described high tibial osteotomies.

Table 1.

Summary of advantages and disadvantages of slope reduction HTO techniques

Technique	ACW-HTO	MOW-HTO	LCW-HTO
Advantages	- Biomechanically advantageous in both knee flexion and extension - Can visualize previous ACL tunnels, allowing for simultaneous bone grafting - Decreased risk of PCL injury. - Able to manipulate tibial tubercle and avoid altering patellar height	- Tibial bone stock preservation - Fibula sparing - Low risk of peroneal nerve injury - Intra-operative adjustments to the osteotomy angle can be made - Can be combined with concomitant ligamentous procedures.	- Natural tendency to decrease the PTS providing multiaxial stability
Disadvantages	- Risk of resultant genu recurvatum - Damage to the popliteal neurovascular structures	- Increased risk of nonunion due to decreased bony apposition and compression	- Increased risk of damage to the peroneal nerve - Intraoperative adjustment of the osteotomy angle is less feasible. - Metadiaphyseal bone loss and changes in tibial condylar offset - Hardware complications

HTO, high tibial osteotomy; ACW-HTO, anterior closing wedge high tibial osteotomy; MOW-HTO, medial opening wedge; LCW-HTO, lateral closing wedge; ACL, anterior cruciate ligament; PCL, posterior cruciate ligament; PTS, posterior tibial slope

Clinical Results

As osteotomy techniques gain popularity as an adjunct procedure in younger patients with chronic recurrent instability or malalignment, increased interest has developed for evaluating clinical outcomes after MOW-HTO and the LCW-HTO, isolated or in combination with ligament reconstruction, in the setting of medial compartment osteoarthritis and ACL deficiency [47–54]. Recent studies evaluating patients with medial compartment osteoarthritis and chronic ACL deficiency undergoing MOW-HTO have reported improvements in both gait biomechanics as well as patient-reported outcomes 5 years post-operatively [55]. Additionally, a nearly 90% rate of return to pre-operative work and a 50% rate of return to sport at higher level than preoperatively have been reported after osteotomy surgery, either the LCW-HTO or MOW-HTO [56]. Similarly, a previous case series found a 91% patient satisfaction rate and significant improvements in clinical and functional stability after LCW-HTO or MOW-HTO in the setting of medial compartment osteoarthritis and chronic ACL deficiency [57].

During the past years, there has been a growing interest in evaluating ACW-HTO for the treatment of chronic or recurrent ACL deficiency and knee instability, resulting in multiple studies investigating this procedure, specifically in the context of a revision ACL setting. One study, performed in 2014 looking at patients with an average PTS of 14° corrected by an average of 5° over a mean 2.5 year follow-up, showed a significant decrease in ATT and reported significantly increased patient-reported outcomes [29]. In a subsequent 2015 study, nine patients were slightly overcorrected to an average PTS of 4.5° and followed over a mean 4-year period, with no recurrent instability or complications recorded [30]. Improved clinical outcomes with little to no concomitant complications after ACW-HTO have since been demonstrated in other studies, illustrating its ability to improve knee stability and restore function. Table 2 lists recent studies and summarizes their findings.

Table 2.

Summary of technique and findings in recent studies [7•, 29, 30, 58•] evaluating outcomes after ACW-HTO

	Sonnery-Cottet et al [29]	Dejour et al [30]	Akoto et al [7•]	Weiler et al [58•]
Patients	5	9*	20	58**
Surgical technique	ACW-HTO, osteotomy through TT	ACW-HTO, osteotomy proximal to TT	ACW-HTO, osteotomy distal to TT + concomitant LET	ACW-HTO (both proximal and distal to tibial tubercle)
Mean pre-operative PTS (degrees)	13.6°	13.2°	15.3°	14.6°
Mean post-operative PTS (degrees)	9.2°	4.4°	8.9°	6.5°
Mean Slope Correction (degrees)	4.4°	8.8°	6.4°	8.1°
Post-operative ATT	Decreased	Decreased	Decreased	Not reported
Complications/graft ruptures during follow-up period	None	None	None	One complication (postoperative hematoma)

ACW-HTO, anterior closing wedge high tibial osteotomy; PTS, posterior tibial slope; ATT, anterior tibial translation. *Denotes total subjects available for analysis (1 lost to follow-up); **denotes total number of ACW-HTO patients

Furthermore, early investigations have sought to evaluate the role of concomitant stabilization procedures in addition to slope reduction osteotomy and ACL-R. This has been demonstrated in one recent study, where patients underwent a slope reduction HTO (15 to nine degrees) with ACL revision, followed by lateral extra-articular tenodesis (LET) in staged fashion. Patients were followed for more than 30 months, over which they were found to have a significant increase in all patient-reported outcomes, a significant reduction in ATT side-side differences, and no residual pivot. Importantly, no patients in this cohort required a revision surgery [7•]. However, these concomitant procedures may confound assessment of knee instability and overall postoperative outcomes, thus limiting conclusions on the efficacy of HTO.

Lastly, there has been an increased interest in investigating the potential utilization of slope reduction osteotomy in the primary ACL setting. While studies assessing the efficacy of HTO in the primary ACL deficiency are limited, preliminary research is promising. A recent study evaluated patients with pathologic tibial slope, anterior tibial subluxation, and chronic medial meniscal deficiency who were treated with slope reduction and ACL-R and followed over a nearly 3-year period. In the 18 patients evaluated, an improvement of patient-reported outcomes, specifically Lysholm, Tegner, and International Knee Documentation Committee (IKDC) scores, was reported. Additionally, ATT side-side differences decreased more than 10 mm, and no residual pivot shift was recorded. Lastly, in this cohort, no graft re-ruptures were recorded [59]. As previously stated, though these results are promising, it is important to note that these patients underwent both ACL-R and HTO, making it difficult to interpret whether patients’ improved stability was due to their ligamentous reconstruction or offloading of the medial compartment due to the HTO (Table 3 provides summary points for slope-reduction HTO).

Table 3.

Take home messages

Exact lateral knee radiographs provide a cost-effective modality to evaluate PTS in primary and recurrent ACL injuries.

Several studies suggest that PTS > 12° may be associated with increased ATT and greater ACL strain.

The indications for a slope-reducing HTO in primary and revision ACL-R are debated, with strong recommendations limited by a paucity of high-level studies.

Various advantages and disadvantages have been described for ACW, MOW, and LCW HTO techniques.

A multifactorial decision-making process is conducted on a case-by-case basis to determine if a slope-reducing HTO should be considered during the management of revision ACL-R.

PTS, posterior tibial slope; ACL, anterior cruciate ligament; ATT, anterior tibial translation; HTO, high tibial osteotomy; ACL-R, anterior cruciate ligament reconstruction; ACW, anterior closing wedge; MOW, medial opening wedge; LCW, lateral closing wedge

Rehabilitation

Postoperative rehabilitation protocols have been inconsistently reported in the previous literature, consisting of mostly expert-level recommendations rather than evidence-based guidelines. In general, early rehabilitation is encouraged following tibial osteotomy. Patients are restricted to non-weightbearing activities for the first 4 weeks after surgery to protect the osteotomy site and are instructed to utilize crutches for 6–8 weeks postoperatively. During this time, rehabilitation regimens composed of active and passive range of motion activities are employed to prevent stiffness. Following this period of non-weightbearing, patients are progressed to partial weight-bearing rehabilitation, barring any delays in rehabilitation or evidence of poor healing during interval radiographic follow-up. During this phase, the focus is mainly on closed chain exercises, which allows for simultaneous activation of antagonistic muscle groups. At 12 weeks postoperatively, patients are cleared for light impact aerobic activity, with the goal of return to prior level of activity and sport at as soon as 4 months after their corrective osteotomy procedure [32, 60].

Conclusion

Increasing evidence suggests that greater PTS is a risk factor for ACL injury and failure of ACL-R. However, proximal tibial morphology is just one component of a multifactorial risk stratification that surgeons encounter in the setting ACL injury. Other concomitant risk factors, including patient demographics, surgical technique, and graft selection, as well as adjunct procedures such as LET have limited the strength of biomechanical and clinical studies investigating PTS with ACL injury, which requires further investigation. Current studies suggest that a slope-reducing HTO may decrease the risk of failure in revision ACL-R for patients with a PTS > 12°.

Compliance with Ethical Standards

Conflict of Interest

Volker Musahl is a paid consultant for Smith & Nephew. Ehab Nazzal, Bálint Zsidai, Oriol Pujol, Janina Kaarre, and Andrew Curley declare that they have no conflict of interest

Human and Animal Rights and Informed Consent

This article does not contain any studies with human or animal subjects performed by any of the authors