Extent of posterolateral tibial plateau impaction fracture correlates with anterolateral complex injury and has an impact on functional outcome after ACL reconstruction

Non-contact anterior cruciate ligament (ACL) injury results from a pivot-shift like mechanism [37], simultaneously damaging the anterolateral complex (ALC) in 50–80% of cases [1, 5, 36]. Accordingly, injury to the ALC has been identified as the most important risk factor for a high-grade pivot-shift in ACL-deficient knees [4, 7]. For this reason, recent studies recommend a combined ACL/ALC reconstruction (with anterolateral tenodesis) in patients with higher preoperative pivot-shift grades [9, 33].

In addition, due to anterior translation of the tibia on the femur and internal tibial rotation, trabecular bone marrow oedema is observed in up to 80% of patients with a complete ACL injury [28, 31]. The severity of bone contusions depends on the amount of energy imparted [17, 18]. A recently published study distinguished between bone marrow oedema and impaction fractures of the posterior aspect of the lateral tibial plateau, which result from an injury of even greater severity [1]. Such displaced impaction fractures were reported with a high prevalence in the setting of ACL tears in more than 50% of cases [1].

However, it is unclear whether posterolateral tibial plateau bone loss affects the outcome of ACL reconstruction, especially when the ALC is not simultaneously addressed.

The purpose of this study was to examine the influence of morphologic variants of tibial plateau impaction fractures (TPIF) on clinical parameters before and after ACL reconstruction, without addressing the ALC. It was hypothesised that increased posterolateral tibial plateau bone loss on MRI may be associated with higher grades of preoperative and postoperative rotational instability, and impaired 1-year functional outcome.

This is a secondary analysis of prospectively collected data from a randomised-controlled study approved by the Institutional Review Board and the local ethical committee (Zurich Cantonal Ethics Commission, 2017-00750).

Posterolateral TPIFs were identified in 49 of 58 acute ACL injury cases (84.5%), of which 20 (34.4%) were classified as type I, 21 (36.2%) as type IIa, seven (12.1%) as type IIb, and one (1.8%) as type IIIb. ALC injury was detected in 31 of 58 (53.4%) cases. Preoperative assessment indicated a high-grade (II and III) pivot-shift in 29% of patients (n = 17). All preoperative data are presented in Table 1, classified by either high or low-grade TPIF.

Multiple regression analysis of predisposing demographic and morphologic factors revealed an increased association of high-grade TPIF with increasing age (p = 0.036), as well as with increased lateral tibial convexity (p = 0.010) (Table 2).

No ACL re-rupture occurred in this study cohort. Nevertheless, there were three (10%) reoperations in the high-grade TPIF group versus one (3%) in the low-grade group (n.s.), on average 9.75 months after index surgery. Reoperation was performed due to meniscus injury (n = 2) or limited range of motion because of intraarticular scarring (n = 1). One patient with a preoperative grade 2a TPIF experienced a further collapse of the posterolateral tibial plateau 9 months postoperatively. Arthroscopic-assisted reduction was performed and the void was filled with cancellous allograft. At the 2-year follow-up, the patient achieved an IKDC score of 83.9 points.

The most important finding of this study was that a relationship exists between posterolateral tibial plateau bone loss and incidence of concomitant ALC injuries. Moreover, patients with increased posterolateral tibial plateau bone loss had a lower Tegner activity score 12 months postoperatively.

Several studies confirmed the contributing role of the ALC in controlling excessive tibial internal rotation in the absence of the ACL [30, 33]. In biomechanical studies, an increased rotational instability could be shown in all degrees of flexion after cutting the ALC in an ACL-deficient knee [26, 34]. A correlation between ALC injury and pivot-shift grade was described in two recent clinical studies [23, 32]. However, the authors of the two studies reported inconsistent results regarding the relationship between bone bruise and ALC integrity after ACL injury. Impaction fractures of the posterolateral tibial plateau result from an injury of even greater severity compared to bone bruise alone. Morphologic variants of TPIF were recently described by Bernholt et al. [1]. However, it is unknown how bone loss of the posterolateral tibial plateau should be interpreted and/or managed while addressing the ACL injury. In a recent metanalysis, Filardo et al. [8] reported that the presence of subchondral fractures and the lateral location of bone bruise correlated with a higher instability and range of motion limitation, and might, therefore, negatively influence the clinical outcome and return to full activity after ACL reconstruction.

The present study, based on a patient cohort investigated before and after ACL without ALC reconstruction, demonstrates the following: first, there is a positive correlation between high-grade posterolateral TPIF and the incidence of concomitant ALC injuries (p < 0.001). Second, the convexity of the lateral tibia plateau is associated with higher graded TPIF. According to Viskontas et al. [37], non-contact ACL injuries result from an anterior translation of the tibia on the femur and internal tibial rotation and cause more severe bone bruising. The contact mechanism predominantly happens due to a valgus force in combination with translation [37]. The combined injury pattern of ALC/ALC with higher-grade TPIF is most likely explained by the extent of the pivoting mechanism, even resulting in increased preoperative tibial internal rotation [15]. Accordingly, the high-grade TPIF group included (insignificantly) more non-contact and less contact ACL injuries compared to the low-grade TPIF group. Moreover, according to the tibia morphology, some knees are probably more susceptible to pivoting than others [14]. However, it is suggestive that excessive pivoting force, while resulting in higher graded TPIF and ALC injury, should also cause increased assessable instability. Nevertheless, no correlation between preoperative pivot-shift and TPIF or ALC injury was found. The pivot-shift test is very examiner-dependent, which was shown by the fact that the degree of the pivot-shift inversely correlated with the size of the knee, measured by the extension and flexion curvature of the femoral condyle, as used for the LCFI [13]. Several previous studies confirmed a correlation between the grade of preoperative pivot-shift and ALC injury [7, 23, 32]. Therefore, this inconsistency should not be weighted too much.

Regarding the functional outcome 12 months after surgery, Tegner scores of the high-grade TPIFs were significantly lower than in the low-grade group (p = 0.04). Compared to the literature, the high-grade TPIF group’s score was even lower compared to 2274 primary ACL reconstructions of the Danish ACL reconstruction registry [22]. This is in accordance with a recently published study by Bernholt et al., confirming inferior postoperative outcomes in high-grade TPIFs 2 years after surgery [2].

Nevertheless, this study is not able to define whether the impaired functional outcome results from residual rotational instability. Even if so, it does not allow any statement to be made about which structure mainly causes the residual rotational instability: osseous (TPIF) or ligamentous (ALC) disintegrity. or a combination of these two. While it is well known that ALC injury, geometry of the lateral tibia plateau [13], and posterior root tears of the lateral meniscus [10] all increase rotatory instability, the effect of posterolateral tibial bone loss in ACL-intact and ACL-deficient knees needs further investigation. Accordingly, indication for treatment of these fractures is still under debate. From the biomechanical aspect, the lack of more than 10% of the posterolateral tibia or depression of more than 2 mm might result in increased rotational instability of the ACL-deficient knee. Previous studies that treated TPIF-graded 2b or higher with fracture fixation reported good clinical outcomes [20, 25]. The number of here reported and higher graded TPIFs is too small to draw reasonable conclusions in this regard. In this cohort, only one patient experienced further collapse of a type 2a TPIF, which is why further investigation is needed. The correct algorithm for treating these cases is, therefore, still unclear. This study’s relevance, however, lies in that high-grade TPIF should raise awareness of potentially increased rotational instability after ACL reconstruction and, therefore, trigger concurrent ALC reconstruction or TPIF fracture fixation [29].

Strengths of this study include the prospective design with excellent documentation of a large amount of clinical and radiographic data. However, the study should be interpreted in light of its potential limitations. First, 12 months is a short follow-up time, allowing only a restricted statement about rates of return to sport or graft failure. However, the goal was to primarily investigate the clinical impact of posterolateral bone loss related to ACL injury, whose clinical relevance could be clearly demonstrated herewith. Next, MRI interpretation of TPIF and status of ALC status could be criticised, as the most reliable modality for diagnosing an ALC injury is subject to ongoing debates [4]. However, all images were reviewed several times by orthopaedic surgeons and a musculoskeletal radiologist, and a consensus was reached in all cases after consultation among co-authors. Therefore, a high degree of accuracy and reliability can be assumed. Finally, some degree of selection bias is suggestive because meniscus tears amenable to fixation were excluded due to the selection criteria of the principle study. Moreover, multi-ligament injuries were excluded as well, although it can be assumed that higher energy is applied to the knee in these cases. Clinical assessment, however, would be less conclusive if multi-ligamentous injuries had also been included.

Increased posterolateral tibial plateau bone loss is suggestive of a combined ACL/ALC injury. Moreover, patients with increased posterolateral tibial plateau bone loss had a lower Tegner activity score 12 months after ACL reconstruction, which is indicative of increased rotational instability after ACL reconstruction.