The key finding of this study was the difference in the patterns of concomitant injuries among patients with isolated PCL-R, combined PCL-R/ACL-R, and isolated ACL-R (Fig. 4). Patients undergoing isolated PCL-R displayed a lower combined rate of concomitant meniscal, chondral, and neurovascular injuries (44%) compared to patients treated either by combined PCL-R/ACL-R (59%) or isolated ACL-R (56%). The results of the study highlight several differences between operatively treated isolated PCL and combined PCL/ACL tears in terms of demographics, injury mechanism, and associated knee injuries.
The demographic characteristics of the investigated patient groups are in agreement with the previous reports, as we further demonstrate that combined cruciate ligament reconstruction patients are generally older than both isolated PCL-R and ACL-R patients [14, 16, 21] and that there are a greater proportion of males undergoing PCL-R/ACL-R compared to isolated ACL-R (65% vs. 57%). The latter finding suggests the more frequent involvement of males compared to females in traffic accidents with high-energy trauma resulting in serious knee injury [14, 16]. The significantly longer median time from injury to surgery in the isolated PCL-R group compared to other groups is explained by the initial non-operative management of isolated PCL injuries often recommended in Scandinavia, followed by an optional delayed PCL-R if bracing and rehabilitation are unsuccessful [18, 23]. Longer preoperative times have been reported to result in more cartilage lesions in PCL-based isolated and combined knee ligament injuries, which may provide an explanation for the comparable proportions of cartilage injuries reported in patients requiring isolated and combined PCL-R in this study [11, 24].
Findings with respect to the injury mechanism of operatively treated PCL tears are concordant with previous Scandinavian knee ligament registry studies, highlighting their predominantly sports-related causes [14, 16]. This provides further evidence that earlier research [7, 22] conducted on small patient populations and in trauma settings may have overestimated the role of traffic-related accidents in the etiology of PCL tears. While sports are responsible for the majority of operatively treated cruciate ligament tears across all patient groups (54–89%), the role of traffic-related mechanisms are more pronounced in PCL-R and PCL-R/ACL-R patients (20% and 27%, respectively). The higher prevalence of traffic-related mechanisms in injuries with operative treatment of both cruciate ligaments compared to isolated tears further supports the notion that these tears require mechanisms with greater energy at trauma compared to those affecting only the PCL.
With respect to concomitant intraarticular injuries involving the articular cartilage and menisci, this study demonstrated a greater proportion of cartilage injuries in the patient groups with isolated (37%) or combined (40%) reconstruction of the PCL compared to the isolated ACL-R group (26%). High-energy associated direct forces to the knee during PCL tears may be a contributing factor to the higher prevalence of articular cartilage lesions observed in these patients. However, a recent investigation from the Danish Knee Reconstruction Registry reported a lower overall rate of cartilage lesions concomitant to PCL-R compared to ACL-R, discordant with the present findings . Of the cartilage injuries in this study, medially localized injuries of the patella and femoral condyle were more frequent in knees requiring PCL-R in contrast to those where only the ACL was reconstructed, which is a pattern reported by earlier studies of PCL-injured knees [9, 19, 24]. Although increase in contact pressure exerted on the medial compartment of the isolated or combined PCL-deficient knee has been implicated in the development of cartilage injury over time [8, 24], the differences in mechanisms leading to PCL and ACL tears may also explain this characteristic contrast in cartilage injury patterns. Moreover, the overrepresentation of neurovascular (9%) and additional ligamentous (28–44%) injuries in the combined PCL-R/ACL-R group further highlights the detrimental role of high-energy forces and direct trauma to the knee, resulting in complex injuries. Conversely, the lower rate of meniscus injury reported in PCL-R patients may be due to the effect of posterior tibial translation during trauma, which unloads the posterior horns of the menisci and prevents injury at this location. Additionally, the rotatory component characteristic of ACL tears may play a less prominent role in the mechanism of PCL tears, resulting in a comparatively lesser strain on the menisci.
The treatment of concurrent meniscus and cartilage injuries in this study displays considerable variation among patient groups. While a higher rate of both medial and lateral meniscus resection compared to repair was reported across all groups, medial meniscus tears accompanying combined PCL-R/ACL-R were repaired at a higher rate compared to those in isolated PCL-R. The converse relationship was demonstrated regarding lateral meniscus injuries, although these differences did not reach statistical significance. Recent registry-based studies aimed at determining the effect of additional meniscus repair or resection concomitant to ACL-R on postoperative patient outcomes report contrasting results [5, 12, 25]. Consequently, the impact of meniscus injury treatment on functional outcomes following knee ligament surgery remains to be clarified. Despite the high prevalence of articular cartilage damage across all groups in this study, only a small proportion of cartilage injuries were treated operatively. The majority of these injuries remained either untreated or treatment was unreported. Postoperative patient-reported outcomes following PCL-R have previously been reported to be inferior compared to those of ACL-R, which may potentially be influenced by the presence and the treatment of concomitant meniscus and cartilage injuries [3, 14, 17].
While PCL tears are frequently the result of trauma involving high-energy forces such as dashboard injuries and falls on the flexed knee , ACL tears are almost exclusively caused by sports-related activity involving internal rotation of the tibia, pivoting, and valgus forces . In the current study, injuries requiring reconstruction of both the PCL and ACL were frequently accompanied by additional tears of the MCL, LCL, or PLC. A recent investigation of combined PCL-Rs involving both the PCL and ACL similarly reported high rates of concurrent MCL (47%) and LCL/PLC (48%) tears . However, the same study reported markedly greater rates of concomitant ligament injuries in isolated PCL-Rs compared to the present study . While the previous research has drawn attention to the frequent incidence of PLC tears in conjunction with PCL tears [13, 21], the present study demonstrates a greater injury rate of the PLC and collateral ligaments in patients requiring combined PCL-R/ACL-R. Furthermore, these results are novel with regards to the high prevalence of concurrent MCL tears in the combined PCL-R/ACL-R patient group (44%) [16, 21]. The high degree of knee instability caused by bicruciate tears and the high-intensity forces in their injury mechanism may potentially explain the frequent injury of the MCL, LCL, and PLC in this subset of patients. While both isolated and combined PCL-R groups contain knees with tears of multiple knee ligaments, the two groups are firmly distinguished by their concomitant injury patterns. This information suggests that dividing combined injuries into subgroups based on the torn knee ligament pattern may be important to consider when assessing outcomes following operatively treated isolated and combined PCL tears.
The main strength of this study is that the SNKLR contains information on the activities leading to cruciate ligament tears and provides a detailed record of concomitant injuries. With a coverage of 90% of all ACL-Rs conducted in Sweden  and a large sample population, the registry enables the reliable comparison of the epidemiology of operatively managed PCL and ACL tears in Sweden. A limitation of this study is that the SNKLR is a surgical registry, which does not systematically collect information from patients with non-operatively treated cruciate ligament injuries. Consequently, the registry-based comparison of epidemiologic differences between operatively and non-operatively treated PCL injuries is currently not feasible. Thus, interpretation of study results is restricted to the context of operatively managed PCL tears. Additionally, the occasional incompleteness of registry data may lead to a misrepresentation of concomitant injuries and their treatment. The current study is based on a Swedish population, leading to some uncertainty with respect to the generalizability of the results. Finally, the reason for operative treatment of the PCL tears in this study is not provided in the registry, subjecting the observed results in this population to confounding by indication.