Evolving evidence in the treatment of primary and recurrent posterior cruciate ligament injuries, part 2: surgical techniques, outcomes and rehabilitation

Abstract Isolated and combined posterior cruciate ligament (PCL) injuries are associated with severe limitations in daily, professional, and sports activities as well as with devastating long-term effects for the knee joint. As the number of primary and recurrent PCL injuries increases, so does the body of literature, with high-quality evidence evolving in recent years. However, the debate about the ideal treatment approach such as; operative vs. non-operative; single-bundle vs. double-bundle reconstruction; transtibial vs. tibial inlay technique, continues. Ultimately, the goal in the treatment of PCL injuries is restoring native knee kinematics and preventing residual posterior and combined rotatory knee laxity through an individualized approach. Certain demographic, anatomical, and surgical risk factors for failures in operative treatment have been identified. Failures after PCL reconstruction are increasing, confronting the treating surgeon with challenges including the need for revision PCL reconstruction. Part 2 of the evidence-based update on the management of primary and recurrent PCL injuries will summarize the outcomes of operative and non-operative treatment including indications, surgical techniques, complications, and risk factors for recurrent PCL deficiency. This paper aims to support surgeons in decision-making for the treatment of PCL injuries by systematically evaluating underlying risk factors, thus preventing postoperative complications and recurrent knee laxity. Level of evidence V.


Introduction
Posterior cruciate ligament reconstruction (PCL-R) techniques have been studied and evolved over the past decades, providing a solid and evidence-based foundation for the operative management of posterior cruciate ligament (PCL) injuries [1,39,41,45,77,88]. Single-bundle (SB) and double-bundle (DB) PCL-R based on the transtibial or tibial inlay technique and implemented by an open, arthroscopically assisted, or all-arthroscopic approach, all offer certain advantages and disadvantages [1,8,15,16,34,40,45,75,77,87,89]. On the other hand, the PCL is characterized by a strong intrinsic healing capability, making the non-operative treatment approach a viable option, especially for partial PCL tears and tibial avulsion injuries of the PCL [2,3,26,71,72]. Good subjective and objective long-term outcomes after non-operative treatment with a fairly low prevalence of 11% of moderate to severe osteoarthritis (OA) after more than 14 years follow-up, keep the debate about the optimal treatment approach, operative vs. non-operative, in PCL injured patients ongoing [71]. As a result, a wide range of viable treatment options is available, enabling an individualized treatment approach based on the injury pattern and the patient's compliance and demands.
In this review, we present the indications, techniques, and outcomes of operative and non-operative treatment of primary and recurrent PCL injuries. Moreover, risk factors associated with recurrent PCL deficiency and future perspectives are outlined.

Indications for PCL-R
Operative treatment of PCL injuries is indicated for patients with symptomatic grade III (complete) tears displaying inadequate functional improvement in response to non-operative treatment. Furthermore, patients with PCL injuries with high-grade knee laxity, or combined with intraarticular or capsuloligamentous injuries should be considered for operative treatment [55,79]. A side-to-side difference in posterior tibial translation (PTT) greater than 8 mm revealed by stress-radiography indicates a complete PCL tear and presents an indication for operative treatment for symptomatic patients [29,68,69]. Additionally, the patient's demands are essential in treatment decisionmaking, leading to the recommendation of PCL-R in an athletic population [39]. There is currently insufficient evidence in the literature to support a definitive treatment protocol; however, the historical preference towards nonoperative treatment of isolated PCL injuries has recently displayed a shift in favor of operative treatment [17,61,79]. Indications for operative treatment may be based on increased translational and rotational tibial movement compared to contralateral PCL intact knees (as measured on in vivo kinematic analyses) [43]. An increasing number of intra-articular injuries (meniscus, cartilage), pathological changes of the anterior cruciate ligament (ACL), and an increased joint contact pressure (tibiofemoral and patellofemoral) have been observed in PCL deficient patients [20].
A failure rate of 1-25% after primary PCL-R is reported, mounting to 45% if unfavorable patient-reported outcomes (Knee Injury and Osteoarthritis Outcome Score < 40 points) are considered as subjective failure [4,36,41,46,81,88]. However, inadequate reporting and varying definitions of failures require caution in the interpretation of failure rates. Debilitating pain and functional impairment during daily activities combined with a PTT of ≥ 10 mm or confirmed graft failure based on magnetic resonance imaging (MRI) scans are accepted indications for revision PCL-R [58,59]. The aim of revision PCL-R is to address the cause of initial failure and eliminate concurrent pathological changes causing pain and instability [41,59]. Therefore, in case of revision PCL-R, extended diagnostic work-up including MRI, hip-knee-ankle radiographs, and computed tomography (CT) scans are required to assess concomitant injuries, lower limb alignment, and prior bone tunnel placement. The requirement of bone grafting due to prior semi-anatomic bone tunnels or the need for corrective osteotomies sometimes require staged revision PCL-R.

Technical aspects in PCL-R
Owing to the anatomy of the PCL and the complex nature of PCL injuries, there is no consensus for a specific operative technique when considering PCL-R. Many biomechanical and clinical research efforts have enabled the development of techniques focusing on anatomic restoration of native knee kinematics [28,66,85]. Variations between the different reconstructive techniques depend primarily on tunnel placement, graft choice, graft positioning and fixation, and the choice of an arthroscopic or open surgical approach (Fig. 1). While studies have reported that both SB and DB PCL-R improve knee kinematics, biomechanical results have recently shown DB PCL-R to closer approximate the native state of the ligament [15,23,85].
The focus of anatomic SB PCL-R is to restore knee kinematics by aligning the path of the PCL graft with the native course of the stronger and more prominent anterolateral bundle (ALB). On the other hand, DB PCL-R aims to provide an alternative method by restoring the synergistic 1 3 functionality of the ALB and posteromedial bundle (PMB) to closer approximate the native anatomy [5,79].
Tibial graft fixation during anatomic SB and DB PCL-R can be performed using the transtibial or the tibial inlay technique by an all-arthroscopic, arthroscopic assisted, or an open approach [8,42,62,76]. Positioning of a PCL drill guide medial to the tibial tubercle just proximal to the pes anserinus, aiming for a sagittal angle of 45°, allows subsequent insertion of a guidewire to reach the anatomic tibial PCL insertion zone when performing the transtibial technique (a more detailed description of radiographic landmarks for the native femoral and tibial insertion zones of the PCL is given in Part 1 of the evidence-based update on the management of primary and recurrent PCL injuries) [18,74]. A frequently reported drawback of the transtibial technique is the formation of an acute angle by the PCL graft exiting the tibial tunnel, also known as the "killer turn" [53]. Graft degeneration, abrasion, and delayed or incomplete graft maturation are attributed to the killer-turn effect, which is believed to cause residual posterior laxity and increase the failure rate after transtibial PCL-R [8,39,84]. Protection of the posterior neurovascular bundle is of utmost importance. One study has shown that the mean distance between the popliteal artery and the posterior tibial cortex 5 mm distal to the joint line was significantly greater at 90° compared to 0° of knee flexion (7.7 ± 3.8 mm vs. 1.6 ± 1.3 mm) [33]. Accordingly, tibial tunnel drilling with fluoroscopic guidance is recommended at 90° knee flexion to prevent injury of the neurovascular structures.
The tibial inlay techniques have been developed to facilitate graft fixation by employing bone troughs for tibial graft insertion and thereby restoring the original PCL anatomy while diminishing PCL graft stress by avoiding the killer-turn effect [8]. The open or arthroscopically assisted approach to the tibial inlay technique requires the patient to be positioned in the lateral decubitus or prone position, or in supine position if the hip and knee are freely movable. After careful dissection, the bone block of the graft can be placed in the created trough and a screw is used for tibial graft fixation [8,42].
Posterior cruciate ligament graft fixation is usually performed using interference screws, or suspensory fixation techniques [14,21,60,79]. To optimize graft loads and to restore native knee kinematics, the ideal knee flexion angle during graft fixation is important [24,31,32]. Consequently, it has been demonstrated that graft fixation angles ranging from 75° to 105° of knee flexion restore knee kinematics to the same extent after SB PCL-R, albeit failing to restore laxity compared to the intact state [31]. In the case of DB PCL-R, PMB graft fixation at 0° and ALB graft fixation at 90° or 105° of knee flexion best restores native knee kinematics while avoiding excessive restriction of the tibiofemoral joint [32].
Graft choice is crucial in PCL-R and includes allografts and autografts as well as soft tissue-only, bone-tendon, and bone-tendon-bone grafts. While soft tissue-only grafts are often preferred for the transtibial technique, tendon-bone grafts are usually used when performing the tibial inlay technique [8,42,67]. Less residual PTT has been demonstrated when using autografts compared to allografts for PCL-R [6,7]. Although a statistically significant difference between autograft and allograft use was demonstrated, the clinical relevance of a mean side-to-side difference in PTT of less than 1.5 mm has to be questioned [6,7,44,80]. The use of allografts was associated with a shorter operation time, while autografts were associated with donor-site morbidity. With respect to patient-reported outcomes and graft failure rates, no difference between allografts and autografts for PCL-R could be demonstrated [6,7]. Therefore, no appreciable difference seems to exist between the usage of allogenous or autologous tissue for PCL-R [25,46,82].

Single-bundle (SB) vs. double-bundle (DB)
Biomechanical studies have demonstrated that only anatomic DB PCL-R is able to restore native knee laxity across the entire range-of-motion and that SB PCL-R leads to residual posterior laxity [31,32,66]. Consequently, several clinical studies have advocated DB PCL-R and have demonstrated improved clinical and functional outcomes compared to SB PCL-R [15,36,45]. A randomized controlled trial comparing isolated SB (n = 22) and DB (n = 24) allograft PCL-R reported significant improvements in patient-reported outcomes (International Knee Documentation Committee (IKDC) Subjective Knee Form, Lysholm Score, Tegner Activity Scale) and reduced knee laxity (KT-1000) for both techniques with a minimum of two-years of follow-up [45]. The DB group showed superior results in knee laxity as well as objective and subjective IKDC compared to the SB group [45]. Another randomized controlled trial showed similar results with significantly less residual PTT measured on posterior stress radiographs in patients undergoing isolated DB (n = 28) PCL-R compared to SB (n = 25) PCL-R [88]. However, the authors questioned the clinical relevance of the statistically significant difference of 1.4 mm between the groups [88]. Accordingly, no difference in clinical and radiological outcomes, failure and survival rates between SB (n = 28) and DB (n = 36) PCL-R could be observed in long-term follow-up (minimum 10 years) [89].
Recently, remnant preservation in SB and DB transtibial PCL-R has gained increasing interest. Preservation of the remnant PCL fibers and meniscofemoral ligaments is believed to provide graft protection by stabilizing the graft and additionally reducing the killer-turn effect by cushioning the graft at the proximal aperture of the tibial tunnel [38,39,78,88]. Therefore, improved graft healing and maturation is assumed by enhanced revascularization and regeneration of mechanoreceptors [38,39]. In one study, improved patient-reported outcomes, extensor and flexor muscle peak torque, functional performance, and decreased PTT, based on posterior sagittal stress radiographs, have been reported in 52 isolated SB transtibial PCL-R with remnant preservation after a mean follow-up duration of 30 months [39]. In 1 3 this study cohort, acute PCL-R (mean time from injury to surgery, 2.4 months) enabled remnant preservation, which resulted in normal graft appearance (signal intensity, gross appearance) in almost 79% of patients as assessed by postoperative MRI (mean time from surgery to MRI, 15.8 months) [39]. Similar results have also been shown for combined PCL and posterolateral corner reconstruction [38].

Tibial inlay vs. transtibial
In the early 1990s, a new technique for tibial graft fixation in PCL-R, termed the tibial inlay technique, was introduced [8,27]. The tibial inlay technique was subsequently advocated to prevent increased graft stress, degeneration, and abrasion caused by the so-called ''killer turn'' at the proximal tibial tunnel aperture in transtibial PCL-R [8]. However, a recently published study reported that remnant preservation in acute transtibial PCL-R enables to avoid the negative influence of the killer turn by a cushioning effect of the remnant PCL fibers [39]. Unfortunately, remnant preservation is not possible in chronic PCL deficiency and revision PCL-R, and, therefore, the tibial inlay technique has been suggested as a viable treatment alternative for such cases [41,42,59]. In spite of a biomechanically confirmed superiority of the tibial inlay technique compared to the transtibial technique in terms of residual posterior tibial laxity and graft degeneration [9], this is not translated into clinical outcomes [40,50,70,75]. Since research has shown that there is no significant correlation between residual posterior tibial laxity and patient-reported outcomes, the biomechanically suggested superiority of the tibial inlay compared to the transtibial technique needs to be questioned [64,73]. In one study, the tibial inlay technique was compared to the transtibial technique in 66 isolated PCL-Rs at a mean follow-up of 148 months [77]. Patients undergoing isolated tibial inlay PCL-R (n = 30) with bone-patellar tendon-bone autograft showed no difference in postoperative patient-reported outcomes (Lysholm Score and Tegner Activity Scale), manual laxity testing (posterior drawer test at 90° knee flexion), instrumented posterior laxity testing (stress radiographs at 90° knee flexion), and progression of OA compared to patients undergoing transtibial PCL-R (n = 36) with hamstring tendon autograft [77]. Consistently, most studies report statistically significant improvement in clinical and functional outcomes postoperatively compared to preoperatively [50,70,77]. However, regardless of the performed tibial fixation technique, considerable rates of residual posterior tibial laxity are reported. One comparative study has shown that 46% and 57% of patients undergoing transtibial and tibial inlay PCL-R, respectively, reported residual episodes of subjective instability [50]. On the other hand, the all-arthroscopic transtibial technique is surgically less demanding, avoids an invasive surgical approach, has a reduced operation time, has a lower risk of complications, and allows for the possibility for remnant preservation [40,70]. However, the tibial inlay technique demonstrates advantages for the treatment of chronic PCL injuries and for the increasing number of revision PCL-Rs [8,41,42,59]. Consequently, future research should focus on identifying specific indications for each technique and thus facilitate surgical decision-making.

Primary vs. revision PCL-R
Failures after primary PCL-R are reported to cause severe impairments in daily living and are a burdening condition for most patients [41,58,59,81]. In such cases, a revision PCL-R is required and is sometimes considered as a salvage procedure. The rate of revision surgery after isolated PCL-R is reported to be 3% and slightly higher (3.4%) after combined PCL-R [46]. However, only a few studies and case reports have reported on the outcomes of revision PCL-R [16,41,42,47,59,81]. In one study, revision PCL-R using quadriceps tendon-bone autografts in a DB technique has significantly improved patient-reported outcomes, activities of daily living, sports activity level, occupational rate, and PTT based on posterior stress radiographs in 15 patients after a mean follow-up of 44 months [59]. Two of the subjects underwent revision PCL-Rs that failed a second time [59]. A recently published study has reported similar results after 22 revision PCL-Rs using Achilles tendon allografts with a DB tibial inlay technique [41]. Patient-reported outcomes and objective evaluation increased significantly while PTT, based on posterior stress radiographs as well as knee laxity (measured by KT-1000 arthrometry), decreased significantly after a mean follow-up of 40 months. Additionally, based on the Tegner Activity Scale, 77% of patients undergoing revision PCL-R were able to return to normal activities of daily living [41]. However, research has shown that 46% of failed PCL operative procedures are not amenable to revision PCL-R mostly due to advanced degenerative changes and OA [58]. Given that revision PCL-R is inherently related to an alteration of the native anatomy and the bony landmarks due to the primary PCL-R, revision surgeries represent more challenging procedures [59]. Therefore, it is recommended to perform a comprehensive diagnostic work-up including patient history, thorough clinical examination, gait analysis, AP, lateral, and weight-bearing hipknee-ankle radiographs as well as MRI and CT scans prior to considering revision PCL-R. Consequently, risk factors related to failures in the operative treatment of PCL injuries can be assessed to facilitate treatment decision-making [58,59].

Failure analysis and survivorship in PCL-R
Unlike in ACL reconstructive procedures, less is known about risk factors and causes of failure in the treatment of PCL injuries [16,41,58,81]. Failed operative treatment of isolated and combined PCL injuries is associated with severe limitations in daily, professional, and sports activities [58,59,81]. One study investigating 52 failed operative PCL procedures (including SB PCL-R, PCL repair, synthetic graft replacement, and PCL thermoplasty) reported that 71% of patients complained of moderate to severe pain during daily activities, 49% described their own knee condition as poor, and 75% have completely quit sports activities after a mean time of 42 months after the failed operative PCL treatment [58]. However, synthetic graft replacement and thermoplasty as a treatment for PCL injuries have been abandoned, which needs to be considered when interpreting the reported poor results. To avoid the detrimental effects of failed PCL-R, it is essential to be aware of the risk factors and understand the underlying causes of failure. In more than 50% of failed PCL-R, multiple factors for failure have been identified [16,41,58]. Posterolateral corner deficiency and femoral or tibial tunnel misplacement have been shown to be the most common causes of failure, accounting for 40-77% and 33-41%, respectively (Fig. 2) [41,58]. An incorrect tunnel placement is characterized by a too proximal (deep) and posterior (low) position on the femoral site or a too anterior and proximal position on the tibial site, resulting in a vertical position of the graft [56,58]. Further risk factors are varus malalignment, primary PCL suture repair, biological and technical failures, wrong surgical decision-making, a too early or too progressive rehabilitation protocol, and a low annual volume of PCL-Rs performed by the operating surgeon [16,58,81].
Recently, a biomechanical study has shown an inverse correlation between the graft force after PCL-R and the posterior tibial slope (PTS). Irrespective of the loading condition and the knee flexion angle, a flatter (reduced) PTS leads to increasing PCL graft forces [10]. Clinically, a significant negative correlation between PTS and residual PTT measured on posterior stress radiographs and a significant positive correlation between PTS and the reduction of PTT from pre-to postoperative has been shown for SB PCL-R [22]. However, this observation has not been demonstrated for DB PCL-R [11]. Additionally, a significantly lower PTS has been observed in patients undergoing primary PCL-R compared to sex-and age-matched controls without ligamentous injury (6° vs. 9°) [12]. Analysis of the injury mechanism revealed that the PTS was significantly lower in non-contact PCL injuries compared to contact injuries (5° vs. 6°) [12], highlighting the impact of the PTS on AP laxity.
At 15 years follow-up with graft failure as the endpoint (need for revision PCL-R, high tibial osteotomy, arthroplasty, complete graft tear based on MRI, or > 10 mm side-to-side difference in PTT based on posterior stress radiographs), the survival rates have been reported to be approximately 82% and 84% for SB (n = 28) and DB (n = 36) Achilles tendon allograft PCL-R, respectively [89].

Complications in PCL-R
Complications in isolated and combined PCL-R have been reported to occur in up to 53% of surgeries and can be divided into complications generally associated with operative procedures and in complications which are inherently related to the different techniques in PCL-R [15,34,36,38,40,45,59,67,70,77,81,88,90]. Well-known complications in knee surgery and thus also described in PCL-R are postoperative hematoma, surgical site infections, arthrofibrosis, reflex sympathetic dystrophy syndrome, anterior knee pain, paresthesia, neurovascular injuries, deep vein thrombosis, and graft failure with recurrent pain and instability [15,16,36,59,67,70,77,81,87,90]. Iatrogenic injuries to the neurovascular structures of the posterior part of the knee represent the most dreaded complications in PCL-R. The occurrence is rare, yet several case reports about popliteal artery lacerations, occlusions, and also popliteal vein injuries have been published [51,57,86]. Precise anatomic knowledge and fluoroscopically guided tibial tunnel drilling in 90° knee flexion may help to prevent iatrogenic neurovascular injuries [33,54]. In the setting of revision PCL-R, scar formation has to be considered, which may alter the natural course of the neurovascular structures.
A meta-analysis reported that the incidence of perioperative complications is 1.7 times higher for the tibial inlay compared to the transtibial technique. However, no statistically significant difference was noted [40]. The open tibial inlay technique requires careful dissection of the popliteal fossa and poses the risk of neurovascular injuries [8,42,81]. Consequently, all-arthroscopic tibial inlay techniques have evolved and have been shown to be biomechanically comparable to an open tibial inlay technique [91]. Additional reported complications related to the tibial inlay technique include fractures of the tibial bone plug during graft fixation [34]. Double-bundle PCL-R is surgically challenging and requires precise knowledge of anatomy and bony landmarks for accurate tunnel placement [5,56]. One study reported a fracture of the separating bony bridge between the ALB and PMB femoral tunnel in DB PCL-R [87]. Furthermore, the tibial inlay and DB techniques are associated with a significantly longer operation time compared to the transtibial and SB techniques, increasing the overall risk of peri-and postoperative complications [45,70].

Non-operative treatment, rehabilitation, and return-to-sports (RTS)
Within the past 10 years (2010s), studies have consistently supported non-operative treatment for isolated grade I, grade II, and nondisplaced tibial avulsion PCL injuries [52,63,65,71,72]. There continues to be a debate regarding the management of isolated grade III injuries as there is limited data on the outcomes following non-operative treatment. A prospective cohort study in high-level athletes with grade II (n = 25) and grade III (n = 21) isolated acute PCL injuries showed that approximately 83% of athletes were able to participate at a competitive sports level (mean Tegner Activity Scale, 9) after non-operative treatment at an average followup period of 5 years [2]. In addition, an epidemiological study demonstrated a median lay-off time of 31 days after PCL injury for professional male soccer players. However, these prospectively collected data in men's professional soccer included all grades of PCL injuries as well as operatively and non-operatively treated athletes [48]. Accordingly, initial non-operative management based on functional bracing and rehabilitation with optional delayed PCL-R seems to be reasonable for isolated acute PCL injuries, even for highlevel athletes with grade III PCL injuries [2]. Although the PCL has a strong intrinsic healing capability, residual posterior laxity is a serious and frequently observed disadvantage of non-operative management [52,71,72]. However, the subjective and objective outcomes after non-operative treatment are promising [3,26,30,63,71,72]. One prospective study demonstrated increased knee laxity based on manual testing in 9% of patients following non-operative treatment after a mean follow-up of 14 years. Additionally, instrumented laxity testing (KT-1000) revealed a mean sideto-side difference of 3 mm [71]. Nevertheless, the majority of patients were able to regain functional range-of-motion (ROM) and sufficient quadriceps strength to return to activities of daily living, with 45% participating in jumping and pivoting activities [71]. Furthermore, no correlation between functional outcomes and grade of laxity could be observed [71]. While non-operative management remains an integral part of the management of isolated PCL injures, it is important to acknowledge that unsatisfactory outcomes may occur. One study showed that patients undergoing non-operative treatment of isolated PCL injuries occasionally experienced pain and swelling in 81% and 56% of patients, respectively [13]. Additionally, a considerable number of PCL deficient patients developed subsequent meniscal injuries requiring subsequent surgery as well as a deterioration of the articular cartilage on average 13 years after the injury, indicating residual knee laxity [13]. This is also supported by the development of moderate to severe OA in approximately 11% of patients at long-term follow-up [71]. There is a paucity of studies comparing operative and non-operative treatment in PCL deficient patients. However, it has been shown that nonoperative treatment leads to significantly more subsequent meniscal injuries as well as a higher rate of OA and a higher conversion rate to total knee arthroplasty compared to operative treatment [83].
Rehabilitation protocols whether for non-operative treatment or postoperative care, are inconsistently reported in the literature [65]. Agreement exists in the combination of temporary immobilization/bracing and exercise therapy. Accordingly, appropriate stabilization by initial static and later functional bracing accompanied by progressive exercise therapy is important, whether post-injury or postoperatively, to support the healing process of the PCL [3,26,30]. A dynamic anterior drawer brace facilitates end-to-end contact between the torn PCL fibers by applying an anteriorly directed force along the proximal tibia [37]. Studies demonstrated a reduction of PTT based on instrumented laxity measurement following non-operative treatment using static and dynamic braces with posterior tibial support [26,30]. Initially, partial weight-bearing is recommended and ROM exercises are performed in the prone position to minimize hamstring activity and to counteract the gravity-induced posterior tibial sag [2,35,65]. The following weeks are accompanied by advancement to full weight-bearing with strong emphasis on quadriceps strengthening. Jogging and sportspecific exercises are often initiated in the sixth postoperative month. Full ROM, quadriceps strength, and a firm endpoint in the posterior drawer test are required before return to cutting and pivoting sports [35,65]. This can take up to 12 months, however, quicker recovery with return to sports at 16 weeks has been reported in high-level athletes [2].
Following PCL-R, weight-bearing as tolerated with a knee brace providing posterior tibial support and locked in full extension is recommended for the first 3-6 weeks, followed by functional bracing for up to 6 months, to promote healing and prevent a fixed posterior tibial subluxation [19,35,65]. The authors' recommendation for non-operative treatment and postoperative rehabilitation is illustrated in Fig. 3.

Future research
It is unknown which demographic, surgical, or patientrelated factors reliably predict clinical and functional outcomes for both operative and non-operative treatment. Since non-operative treatment may yield satisfactory outcomes, an initial non-operative treatment of PCL injuries with an optional delayed PCL-R is often recommended. However, the optimal timing of operative PCL-R (early vs. delayed surgery) as well as the most appropriate timing of postinjury/postoperative rehabilitation (early vs. delayed) is not sufficiently supported by high-quality evidence, which is even more pronounced in combined PCL injuries. Therefore, a prospective randomized multi-center clinical trial-Surgical Timing and Rehabilitation (STaR) Trial for Multiple Ligament Knee Injuries-is currently ongoing to provide evidence for the optimal timing of operative treatment and non-operative/postoperative rehabilitation [49].

Conclusion
As demonstrated in this two-part review PCL injuries are complex and commonly associated with neurovascular compromise and multiple ligament knee injuries involving the PLC and less commonly MCL. Clinicians must have a thorough understanding of anatomy and biomechanics to aid in decision-making, analysis of concomitant injuries, and operative treatment. Treatment algorithms following history and physical examination involve advanced imaging including stress radiographs and MRI. Exact treatment decisions are finalized and often revised after examination 1 3 under anesthesia and afford availability of multiple surgical tools and graft choices as well as the flexibility of the experienced surgeon. Good to excellent outcomes with high patient satisfaction can be achieved. High-quality and largescale studies are needed to provide further evidence for an individualized treatment approach pursuing the ultimate goal of restoring native knee kinematics and facilitating a return to daily, professional, and sports activities.
Author contributions All listed authors have contributed substantially to this work: PWW, BZ, NNW, AH and JDH performed the literature review and primary manuscript preparation. VM, EHS and KS assisted with literature review, initial drafting of the manuscript, as well as editing and final draft preparation. All authors read and approved the final manuscript.
Funding Open Access funding enabled and organized by Projekt DEAL.

Compliance with ethical standards
Conflict of interest VM reports educational grants, consulting fees, and speaking fees from Smith & Nephew plc, educational grants from Arthrex, is a board member of the International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports Medicine (ISAKOS), and deputy editor-in-chief of Knee Surgery, Sports Traumatology, Arthroscopy (KSSTA). In addition, VM has a patent Quantified injury diagnostics-U.S. Patent No. 9,949,684, Issued on April 24, 2018 issued to University of Pittsburgh.
Ethical approval Not applicable.

Informed consent Not applicable.
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