Anatomy and function of the PCL of the knee

Written by George Dowle | Dec 4, 2024 4:16:10 PM

Knees are a common source of pain and injury in both athletes and general population. The knee has a complex configuration of bony and soft tissue structures that work together to provide movement and stability during static and dynamic situations. The Posterior Cruciate Ligament (PCL) is one of the cruciate ligaments of the knee along with the more commonly known Anterior Cruciate Ligament (PCL). This review will briefly describe the role/function and also mechanism of injury of both PCL and ACL and then focus more on the anatomy, biomechanics and management of PCL injuries.

Roles/Function of the cruciate ligaments:

  • PCL - provides restraint to the posterior translation of the tibia relative to the femur
    •  PCL also acts as a secondary restraint to resist varus, valgus, and external rotation moments about the knee
  • ACL - provides restraint to the anterior translation of the tibia relative to the femur

Mechanism of injury:

  • PCL -  posterior force directed on the tibia, most common with the knee in a flexed position
  • ACL - Majority non-contact injuries during a cut-and-plant movement with change in direction or speed e.g deceleration, landing from a jump, twisting/pivoting is the typical mechanism that causes the ACL to tear, being a sudden change in direction or speed with the foot firmly planted

Relevant anatomy of the PCL:

  • Origin:  anterolateral aspect of the medial femoral condyle within the notch
  • Insertion:  posterior aspect of the tibial plateau
    • The PCL is comprised of two bundles: the larger anterolateral bundle (ALB) and smaller posteromedial bundle (PMB)
    •  The two bundles of the PCL insertion to the tibia as a conjoined structure
      • The long-held notion was that the two bundles functioned independently, with the ALB acting primarily during flexion and the PMB in knee extension, however, more recent research suggests that the two bundles function synergistically
      • Multiple studies have demonstrated that an isolated tear of either bundle does not result in a clinically significant increase in posterior translation of the tibia
      • In the setting of knee flexion, the ALB is tight and the PMB is lax, whereas in extension, the ALB is lax and the PLB is
         tight
      • The majority of the strength of the PCL comes from the ALB, as the tensile strength of the ALB is 1620 N while the tensile strength of the PMB is 258 N. The difference in strength is largely attributed to the significant variation in cross-sectional area of these two bundles (43 and 10 mm2)
    • PCL is innervated by branches of the tibial nerve while its vascular supply comes from the middle geniculate artery 
    • The thickness of the PCL is nearly twice that of the ACL

Function:

  • Prevents excessive posterior translation of the tibia on the femur
  • Also secondary stabilizer against excessive rotational forces specifically between 90° and 120° of knee flexion
    • PCL plays a larger role in rotational stability than previously assumed, especially beyond 90° of flexion
    • While patients may tolerate a PCL-deficient knee, it has been reported that the PCL-deficient knee results in distorted loads and different kinematics during functional activities

Other relevant anatomy:

  • Combined PLC injuries have much influence on the prognosis of PCL injury treatments. The PLC serves as the primary restraint to both varus and external rotation forces, with the PCL acting as a secondary restraint
  • The Posterolateral compartment (PLC) can be described as consisting of 5 structures (2 muscles and 3 ligaments);
    • the lateral head of the gastrocnemius muscle
    • the popliteus muscle
    • the popliteofibular ligament
    • the lateral collateral ligament (LCL)
    • the arcuate ligament–fabellofibular ligament complex.
      • The biceps femoris tendon and iliotibial band also contribute to the stability of the PLC of the knee, and may be damaged with injuries in this region
    • Disruption of the PLC with an intact PCL results in increased varus and external rotation of the knee, most pronounced at 30° of knee flexion, while disruption of the PCL with an intact PLC results in increased posterior translation of the tibia, most pronounced at 90° of knee flexion. 
      • Therefore during the dial test:
        • Excessive external rotation at 30 degrees only = PLC injury
        • Excessive external rotation at 90 degrees only = PCL injury
        • Excessive rotation at both 30 and 90 degrees = PCL and PLC injury

Treatment:

  • Conservative treatment is indicated for PCL injuries with 5 to 10 mm posterior instability (grade I and II)
    • Key component = Quads strengthening
      • The function of the quadriceps to extend the knee, and due to its attachment site on the anterior side of the base of the tibial plateau, its accessory motion pulls the tibia forward on the femur when extending the knee, pulling in the opposite direction of the PCL’s function. In contrast, hamstrings are responsible for flexion of the knee, and would increase posterior sheer force from the tibia thus stressing or re-tearing a healing PCL
  • Surgical treatment is recommended for PCL injuries with ≥10 mm posterior instability (grade III) or with combined collateral ligament injuries or avulsion fractures
    • Single bundle reconstruction: The Single Bundle Reconstruction (SBR) recreates the largest and stronger bundle of the two – The ALB. The ALB is the stronger bundle and is the primary restraint to posterior translation. However, without the two bundles, they do not have as much support in full flexion when compared to a double bundle reconstruction. SBR provides full anteroposterior support from 0-60° of flexion. Most people do not require that much flexion in everyday activities. 
    • Double bundle reconstruction: attempts to better recreate the native PCL with the ALB and PMB. There is evidence that this method better recreates their codominant relationship thus restores the knee to greater stability for resisting posterior tibial translation than SBR. Shown to have less laxity and posterior tibial translation however compared to the native knee has more posterior tibial translation between 0-15 degrees flexion therefore does not quite have the same restraints as a native PCL.  DBRs are usually considered for high performance athletes because they may need the stability in all degrees of flexion. DBR is less favoured in patients with osteoporosis as fewer tunnels reduces fracture risk - SBR or conservative. 
    • Considerations
      • Natural history - Short-term results of nonoperative treatment have been reported successful in many 
        studies. However, there are studies showing unfavorable long-term results including degeneration of the tibiofemoral cartilage in the medial compartment and increased tibiofemoral pressure and meniscal strain, which eventually led to arthritis. Degenerative arthritis was observed in 80% of the patients after non-operative treatment
      • Healing potential -  when PCL injuries are combined with PLC injuries, the PCL should be treated with reconstruction 
        and the PLC with repair or reconstruction because the PLC tears do not heal with conservative treatment, which eventually leads to instability and cartilage degeneration
      • Remnant -  Remnants are observed in most PCL injuries because the ligament has a high chance of spontaneous healing. It has been known that preservation of the remnant fibers during operative treatment of PCL injuries plays a pivotal role in obtaining successful outcomes
      • Alignment -  Left untreated, varus malalignment may increase the risk of treatment failure after PLC and PCL reconstruction. Therefore, if varus malalignment is present, an osteotomy should be carried out even in acute cases of PCL combined with PLC injuries
        • A varus osteotomy may improve posterolateral instability and make PLC reconstruction unnecessary
        • Improvements can be made with varus malalignment correction alone and then subsequent reconstruction procedures may become unnecessary

References:

1) Posterior Cruciate Ligament: Anatomy and Biomechanics (2018) - Stephanie L. Logterman1 & Frank B. Wydra1 & Rachel M. Frank

2)  Rupture of Posterior Cruciate Ligament: Diagnosis and Treatment Principles (2011)  Beom Koo Lee, MC and Shin Woo Nam, MC

3) https://www.physio-pedia.com/Posterior_Cruciate_Ligament_Injury