Posterior cruciate ligament injuries

                                    Dr. KS Dhillon

Anatomy and Biomechanics

The posterior cruciate ligament (PCL) length averages between 32mm and 38mm from its origin to insertion, and its cross-sectional area narrows distally [1]. The PCL is mainly made up of two bundles, namely the anterolateral bundle (ALB) and posteromedial bundle (PMB). The femoral attachment of the ALB is nearly twice as big as the tibial insertion. The ALB is more lax in full extension and tight in full flexion. This bundle comprises about 65% of the total mass of the PCL. The PMB is taught in extension and lax in flexion. 

The PCL extends from the posterior tibial plateau to the roof of the femoral notch. It is intraarticular but extra synovial. The synovial membrane covers the entire ligament except for the posterior part. The PCL femoral insertion is half-moon shaped and the PCL tibial insertion is nonplanar and rectangular in shape.

The PCL is surrounded by the anterior meniscofemoral ligament known as the Humphrey ligament and the posterior meniscofemoral ligament which is known as the Wrisberg ligament, which emerges from the lateral aspect of the medial femoral condyle and inserts distally close to the posterior horn of the lateral meniscus. Both these ligaments are only present in 49% of the knees, providing up to 71% of the posterior stability in a PCL-deficient knee, particularly between 60° and 90° of flexion [2].

The PCL is the primary restraint to posterior tibial translation on the femur and the secondary restraint to external rotation [3]. These functional roles are most when the knee is flexed to 90 degrees. When the knee is in full extension there is a minimal posterior translation with an isolated PCL lesion [3]. 

Studies show that kinematic differences increased significantly with combined PCL/posterolateral corner (PLC) lesions when compared with isolated lesions [4,5,6]. 

Mechanism of Injury

Three mechanisms have been proposed for rupture of the PCL:

1. Hyperflexion

Hyperflexion knee injury is common in sports. The individual falls on hyperflexed knee and the traumatic forces are directed proximally up the tibia which leads to posterior subluxation of the tibia and rupture of the PCL.  Nearly always an anterior isolated intrasynovial intrasubstance tear occurs and often the posteromedial bundle remains intact. The laxity is rarely greater than grade II. These lesions can heal to some extent and with time the laxity may often decrease to grade I [7].

2. Pretibial Trauma

The most common traumatic mechanism for injury to the PCL is the dashboard injury. The knee is in a flexed position and a posteriorly directed force is applied to the anterior aspect of the proximal tibia. Such an injury results in an intrasubstance tear at the level of the tibial plateau or a tibial avulsion. Severe trauma will result in damage to the meniscofemoral ligaments as well.

An anteromedial and a rotational force can cause a tear of the posterolateral corner. When the PCL and posterolateral corner is torn, the posterior instability will be greater. 

3. Hyperextension

Hyperextension injury can result in tearing of the PCL and the posterior capsule. Hyperextension can lead to dislocation of the knee and neurovascular compromise. The PCL injury is frequently a proximal disruption at the femoral attachment. 

Epidemiology

The incidence of PCL injuries in the literature varies widely and this disparity results from differences in the population of individuals examined. Miyasaka and Daniel[8] reported an incidence of 3% in the general population, whereas Fanelli and Edson [9] reported an incidence of 38% in patients with acute knee hemarthrosis in an emergency room.

There is an association between athletic injuries and `isolated' PCL injuries. The most frequent mechanism of injury in athletes is hyperflexion injury. In an emergency room setting, 56.5% of injuries are trauma-related and 32.9% are athletic-related [7]. Fanelli and Edson [9] reported that 96.5% of the PCL injuries occurred in combination with other ligamentous injuries. 

The majority of PCL tears occurred in motor vehicle and motorcycle accidents. They concluded that the incidence of PCL injuries is higher in patients with trauma as compared to patients with athletic injuries.

Most grade III PCL injuries occur with other ligament injuries (79% of cases) [10]. Isolated PCL injuries are rare and occur in 3.5% of the patients, whereas 96.5% of PCL injuries occurred in combination with other ligament injuries [9]. 

Classification

The appropriate treatment of PCL injuries remains uncertain. This is partly due to inaccurate classification of patients in follow-up studies. PCL injuries can be classified according to the structures damaged (isolated vs combined), the degree of instability, and the mechanism of injury [7]. 

• Type I- Where the PCL is stretched, laxity is less than 5mm and the tibial plateau is 5mm to 10mm anterior to the femoral condyle.
• Type II- Where the PCL is torn, MF ligaments intact, laxity is 5mm to 9mm and the tibial plateau is 0 to 5mm anterior to the femoral condyle.
• Type III-Where the PCL and MF ligaments are torn, laxity is > 10mm and tibial plateau is flush with the femoral condyle.
• Type IVA- Where there is PCL, LCL, and posterolateral injury, laxity is >12 mm and the tibial plateau is > 2mm posterior to the femoral condyle.
• Type IVB-  Where there is PCL, MCL, and posteromedial injury, laxity is >12 mm and the tibial plateau is > 2mm posterior to the femoral condyle.
• Type IVC- Where there is PCL and ACL injury, laxity is >15mm and the tibial plateau is > 5mm posterior to femoral condyle.

Grades I to III are isolated injuries and grade IV is a combined injury.

Clinical Evaluation

History

A thorough history including the mechanism of injury is useful. The chief complaint and level of activity need to be determined. Patients with acute isolated PCL tear will complain of mild swelling, pain and inability to bear weight. Patients with chronic PCL laxity complain of pain. The pain is most prevalent with long distance walking and descending stairs. The pain is predominantly felt in the retropatellar area and medial compartment of the knee. Other complaints include difficulty walking with the knee extended in mid stance. They also can have apprehension while descending stairs because they get a sense of unsteadiness or sliding of the joint when going downstairs. Athletes often complain of a decreased ability to rapidly change direction [7]. However, significant giving way of the knee or buckling that is commonly seen with an ACL deficient knee is usually not seen with an isolated PCL tears. Instability is common in patients with combined injury.

Physical examination

A physical examination for a posterior cruciate deficient knee following an acute injury must include inspection over the anterior tibia for abrasions, ecchymosis or lacerations, range of motion, gait assessment, and a neurovascular examination. 

Testing for an acute posterior cruciate ligament injury is usually difficult because of a tense hemarthrosis and pain. The most accurate test for determining a posterior cruciate rupture is the posterior drawer test at 90° of knee flexion. Following an acute injury, it is often difficult to achieve 90 degrees of knee flexion because of pain. Staubli and Jakob [11]described significant posterior translation of the tibia at 10 to 15° of flexion. The posterior tibial sag and Lachman test and reverse pivot shift tests can be useful.

Injuries to other ligamentous structures must be excluded. Examination of the ACL, collateral ligaments, and posterolateral corner must be carried out.

Slightly increased posterior translation at 30° but not 90° indicates a posterolateral injury. An isolated PLC injury increases external rotation at all angles of flexion, but this is maximal at 30°. Neurovascular examination must be carried out to exclude peroneal nerve and vascular injury.

Ancillary Tests

Radiographs are taken to rule out an avulsion fracture of the PCL at its tibial insertion. Fibular head fractures are indicative of a PLC injury. Magnetic resonance imaging (MRI) has been shown to be highly accurate in the diagnosis of a complete PCL tear. Associated collateral ligament, meniscal and chondral injuries can also be diagnosed with an MRI. A good history and physical examination is very accurate in diagnosing a PCL injury, and hence an MRI is not routinely required. 

Treatment of isolated PCL injuries

There is no controversy about the treatment of PCL avulsion fractures, which are ideally treated by open reduction and internal fixation when the fragment is large enough to be fixed with a screw. Isolated injuries of the PCL usually produce a grade I to II posterior laxity of the knee while a grade III laxity is produced by a combined PCL and posterolateral corner injuries. What is the best method of treatment of isolated PCL injuries?

An acute midsubstance tear of the PCL may heal [12] unlike ACL mid-substance injuries which do not heal. Logically such injuries should be treated conservatively. The long-term natural history of isolated PCL injuries suggests that there is no indication for surgical treatment of such injuries because the outcome of conservative treatment is good. There are no studies to show that surgical treatment is better than conservative treatment for isolated tears of the PCL. Neither is there any evidence that surgical treatment reduces the incidence of osteoarthritis of the knee after PCL injuries [13]. 

Natural history of isolated PCL injuries

One of the earlier studies of the long-term outcome of non-operatively treated isolated PCL injuries was done by Parolie and Bergfeld in 1986 [14]. They studied 25 patients who were treated conservatively for isolated PCL injuries and followed up for a mean of 6.2 years (2.2 to 16 years). They found that 80% of the patients were satisfied with their knee and 84% had returned to their previous sport, with 68% at the same level of performance and 16% at a reduced level of performance. Patients who were satisfied with their knee and had returned to sports had quadriceps strength of more than 100% of the contralateral uninvolved knee and those who were dissatisfied with their knee and not returned to sports had less than 100% strength of the quadriceps as compared to the contralateral knee. 

Selbourne et al [15] prospectively studied the natural history of acute, isolated, nonoperatively treated PCL injuries in athletically active patients. The study included 133 patients all of whom completed a yearly questionnaire for an average of 5.4 years (2.3 to 11.4 years) and 51% of the patients (68 out of 133) returned for long-term clinical and radiological examination. The functional outcome was good with a mean modified Noyes knee score of 84.2 points, a mean Lysholm score of 83.4, and a mean Tegner activity score of 5.7. The grade of laxity had no correlation with subjective functional outcome. Fifty percent of the patients returned to the same sport at the same or higher level of performance, one-third (33.3%) returned to the same sport at a lower level and one-sixth (16.6%) did not return to the same sport.

The authors concluded that ‘athletically active patients with acute isolated posterior cruciate ligament tears treated nonoperatively achieved a level of objective and subjective knee function that was independent of the grade of laxity’.

The longest follow-up study of patients with acute, isolated PCL injuries treated non-operatively was reported by Shelbourne et al in 2013 [16]. The study included 68 patients who had a subjective follow-up at a mean of 17.6 years and 44 of the patients had a subjective and objective follow-up at a mean of 14.3 years (10-21 years). The mean quadriceps strength was 97% of the contralateral side and the range of knee motion was normal in all patients. 

Fifty percent of the patients had no osteoarthritis (OA) of the knee, 30% had mild OA, 9% (4 patients) had moderate and 2% (1 patient) had severe OA of the knee at a mean follow-up of 14.3 years. The mean IKDC (International Knee Documentation Committee) and modified CKRS (modified Cincinnati Knee Rating System) subjective scores were 73.4 ± 21.7 and 81.3 ± 17.4, respectively at 17 years follow-up and the subjective scores did not correlate with the degree of PCL laxity.

The authors concluded that long-term follow-up of patients, with isolated PCL injuries treated non-operatively, shows that patients remain active, have good muscle strength, full range of knee motion and they report good subjective scores, and that the incidence of post-traumatic OA is low.

In 2007 Patel et al [17] published a study involving 57 patients with acute isolated PCL injuries who were treated non-operatively with a mean follow-up of 6.9 years (2 to 19.3 years). Seventeen patients (29.8%) had a grade I and 41 patients (71.9%) had a grade II laxity of the PCL. The functional outcome at 7 years was good with a mean Lysholm-II knee score of 85.2 points (range 51 to 100 points) and a mean Tegner activity level of 6.6 (range 3 to 10). The Lysholm-II knee scoring system showed excellent results in 40%, good in 52%, fair in 3%, and poor in 5% of the knees. The incidence of mild medial compartment OA was 12% and moderate medial compartment OA 5% and none of the patients had severe OA.

These medium and long-term studies of the natural history of isolated PCL injuries reveal that the subjective functional outcome is good, without surgical intervention, with the majority of the patients returning to their pre-injury activity level. Are the results of PCL reconstruction better than conservative treatment?

Surgical treatment of isolated PCL injuries

Most of the published studies reporting the outcome of PCL reconstruction for isolated PCL injuries are small case series with a short follow-up, and the heterogeneity of the patients studied and the technique used makes it difficult to judge outcomes in these patients [18].

However, there are two studies that have reported the long-term outcome of PCL reconstruction in patients with isolated PCL injuries. 

Herman et al [19] studied 25 patients (22 male, 3 female) with an average age of 30.8 years who underwent single-bundle PCL reconstruction for pain and functional instability of the knee. The mean follow-up was 9.1 years (6.5 to 12.6 years). Twenty-two patients were evaluated clinically and 3 patients were provided telephone interviews.

The final mean IKDC score was 65, Lysholm score was 75 and the VAS (visual analog score) was 8. The functional scores were fair to good and were significantly better than the pre-operatively scores. The final Tegner score was 5.7. The functional results were significantly better in patients with no cartilage damage at the time of surgery and in those who underwent surgery within 1-year post-injury.

Jackson et al [20] evaluated the long-term outcome of PCL reconstruction in 26 patients after failed conservative treatment. At 10 years follow-up, the IKDC score was 87 and the Lysholm score improved from 60 to 90 postoperatively. Twenty-two patients had a radiological examination and 18% of the patients had grade II OA changes and another 18% had grade III OA changes.

A careful analysis of the subjective outcome reported by Shelbourne [12] and Patel [17] for non-operative treatment and that by Herman [19] and Jackson [20] for surgical treatment of PCL injuries appears to be very similar. 

Complications of PCL surgery

PCL injuries are rare and the indications for surgery of the PCL are limited since conservative treatment has a good outcome, therefore the number of PCL surgeries carried out by surgeons per year are small. This limited experience with such complex surgery can lead to a higher incidence of complications especially when there are vital neurovascular structures at the back of the knee. Furthermore catastrophic complications usually never get reported which can give surgeons a false sense of relative risks involved when undertaking such procedures. 

Besides the standard complications, such as those associated with anesthesia and complication of surgery such as infections and thromboembolic complications which can occur with any orthopedic procedure, there are specific complications associated with PCL surgery, some of which includes neurovascular injury, osteonecrosis, fractures, stiffness, residual laxity and anterior knee pain. 

A survey of the frequency of complications associated with arthroscopic surgical procedures of the knee reported by Salzler et al [21], where the data was obtained from the ABOS (American Board of Orthopaedic Surgery) database, showed that the complication rate was the highest for PCL surgery as compared with other arthroscopic procedures. The complication rate for PCL surgery was 20.1%, ACL surgery 9.7%, meniscal repair 7.7%, meniscectomy 2.8%, and chondroplasty 3.5%. The overall pulmonary embolism rate was 0.11% and the infection rate 0.84%. These were self-reported complication rates and the authors believe that the actual rates may be higher.

Conclusion

Posterior cruciate injuries can be caused by a variety of different mechanisms. The usual cause is trauma and sports. It is important to differentiate isolated PCL injuries from combined ligament injuries. The diagnosis can be established by a good history and good clinical examination. If the diagnosis is uncertain an MRI will be useful. It is critical to rule out neurovascular injuries in patients who have combined injuries.

Most patients with isolated PCL injuries can be treated conservatively. The mainstay of treatment is quadriceps strengthening with avoidance of active hamstring exercises. 

Medium and long-term studies of the natural history of isolated PCL injuries reveal that the subjective functional outcome is good, without surgical intervention, with the majority of the patients returning to their pre-injury activity level.

The incidence of posttraumatic OA is low after PCL injuries. The incidence of mild medial compartment OA is 12% and moderate medial compartment OA 5% and none of the patients usually develop severe OA.

A careful analysis of the subjective outcome of non-operative treatment and  surgical treatment of PCL injuries appears to be very similar. 

The complication rate following  PCL surgery is the highest as compared with other arthroscopic procedures. The reported complication rate for PCL surgery is about 20% but in actual fact may be higher.

 

 References

1. Girgis FG, Marshall JL, Monajem A. The cruciate ligaments of the knee joint. Anatomical, functional and experimental analysis. Clin Orthop Relat Res. 1975 Jan-Feb;(106):216-31. doi: 10.1097/00003086-197501000-00033. PMID: 1126079.
2. Gupte CM, Bull AMJ, Thomas RD, Amis AA. The meniscofemoral ligaments: secondary restraints to the posterior drawer. Analysis of anteroposterior and rotary laxity in the intact and posterior-cruciate-deficient knee. J Bone Joint Surg [Br] 2003;85-B:765-773.
3. Castle Jr TH, Noyes FR, Grood ES. Posterior tibial subluxation of the posterior cruciate-deficient knee. Clin Orthop 1992; 284: 193-202.
4. Veltri DM, Deng XH. Torzilli PA, et al. The role of the cruciate and posterolateral ligaments in stability of the knee: a biomechanical study. Am J Sports Med 1995; 23: 436-43.
5. Noyes FR, Stowers SF, Grood ES, et al. Posterior subluxations of the medial and lateral tibiofemoral compartments: an in vitro ligament sectioning study in cadaveric knees. Am J Sports Med 1993; 21: 407-14.
6. Harner CD, Vogrin TM, Höher J, Injury to the posterolateral structures has profound effects on the function of the human posterior cruciate ligament [abstract]. Trans Orthop Res Soc 1998; 44: 47.
7. Janousek et al. Posterior Cruciate Ligament Injuries of the Knee Joint. Sports Medicine 2000, 28(6):429-41.
8. Miyasaka KC, Daniel DM. The incidence of knee ligament injuries in the general population [abstract], Am J Knee Surg 1991; 4: 3-8.
9. Fanelli GC, Edson CJ. Posterior cruciate ligament injuries in trauma patients: II. Arthroscopy 1995; 11 (5): 526-9.
10. Becker EH, Watson JD, Dreese JC. Investigation of multiligamentous knee injury patterns with associated injuries presenting at a level I trauma center. J Orthop Trauma 2013;27:226-231.
11. H-U. STAUBLI, R. P. JAKOB. Posterior instability of the knee near extension. A clinical and stress radiographic analysis of acute injuries of the posterior cruciate ligament. The Bone & Joint Journal. 1990, 72(2):225-30.
12. Shelbourne KD, Jennings RW, Vahey TN. Magnetic resonance imaging of posterior cruciate ligament injuries: assessment of healing. Am J Knee Surg 1999; 12:209-13.
13. Dowd GSE. Reconstruction of the posterior cruciate ligament: Indications and results. J Bone Joint Surg [Br] 2004; 86-B: 480-91.
14. Parolie JM, Bergfeld JA: Long-term results of nonoperative treatment of isolated posterior cruciate ligament injuries in the athlete. Am J Sports Med 1986; 14:35-38.
15. Shelbourne KD, Davis TJ, Patel DV. The natural history of acute, isolated, nonoperatively treated posterior cruciate ligament injuries. A prospective study. Am J Sports Med. 1999 May-Jun; 27(3):276-83.
16. Shelbourne KD, Clark M, and Gray T. Minimum 10-Year Follow-up of Patients after an Acute, Isolated Posterior Cruciate Ligament Injury Treated Nonoperatively. Am J Sports Med July 2013 vol. 41 no. 7 1526-1533.
17. Patel DV, Allen AA, Warren RF, Wickiewicz TL, Simonian PT. The Nonoperative Treatment of Acute, Isolated (Partial or Complete) Posterior Cruciate Ligament-Deficient Knees: An Intermediate-term Follow-up Study. HSS J. 2007 Sep; 3(2): 137–146.
18. Montgomery SR, Johnson JS, McAllister DR, Petrigliano FA. Surgical management of PCL injuries: indications, techniques, and outcomes. Curr Rev Musculoskelet Med. 2013 Jun; 6(2): 115–123.
19. Hermans S, Corten K, Bellemans J. Long-term results of isolated anterolateral bundle reconstructions of the posterior cruciate ligament: a 6- to 12-year follow-up study. Am J Sports Med. Aug 2009; 37(8):1499-507.
20. Jackson WF, van der Tempel WM, Salmon LJ, Williams HA, Pinczewski LA. Endoscopically-assisted single-bundle posterior cruciate ligament reconstruction: results at minimum ten-year follow-up. J Bone Joint Surg Br. 2008 Oct; 90(10):1328-33. 
21. Salzler MJ, Miller CD, Lin A, Irrgang JJ, Harner CD. Complications Following Arthroscopic Knee Surgery. Orthopaedic Journal of Sports Medicine, September 2013 vol. 1 no. 4 suppl 2325967113S00044.