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Editorial Commentary: The Posterior Cruciate Ligament Posteromedial Bundle Is Small but Vital to Posterior Cruciate Ligament Biomechanics: Don’t Ignore the Underdog

      Abstract

      Posterior cruciate ligament (PCL) reconstruction leads to outcomes less favorable than those of anterior cruciate ligament reconstruction. In recent years, we have seen a surge of publications regarding PCL anatomy, isometry, and reconstruction techniques. PCL reconstruction has been revolutionized with lessons learned from analysis of PCL behavior, such as the distinct role of the posteromedial bundle (PMB) in the biomechanics of the knee at different flexion angles, as well as its co-dominant role with its counterpart, the anterolateral bundle. With the knee in extension, the PMB serves to restrict posterior translation, whereas in knee flexion, the PMB restricts internal rotation. It is rather too early to know whether the biomechanical advantage of double-bundle reconstruction will result in better clinical outcomes in the long term; however, the increased interest and the refinement of both single- and double-bundle reconstruction techniques will certainly advance our knowledge, ultimately translating into better patient outcomes.
      Optimization of surgical techniques is key for clinical success. In the past, posterior cruciate ligament (PCL) injuries were treated conservatively, but the development of early osteoarthritis and alteration of patellofemoral biomechanics have led to the recognition of PCL reconstruction as a viable option for complete PCL injuries.
      • Torg J.S.
      • Barton T.M.
      • Pavlov H.
      • Stine R.
      Natural history of the posterior cruciate ligament-deficient knee.
      Outcomes after PCL reconstruction remain generally inferior to those of anterior cruciate ligament reconstruction.
      • Lee Y.S.
      Editorial Commentary: Biomechanics of posterior cruciate ligament tibial fixation.
      Despite the fact that the biomechanical advantage of double-bundle PCL reconstruction was recognized early, it was only recently that clinical data showing superior International Knee Documentation Committee and stability scores for double-bundle reconstruction reignited a conversation about improving our PCL reconstruction technique.
      • Chahla J.
      • Moatshe G.
      • Cinque M.E.
      • et al.
      Single-bundle and double-bundle posterior cruciate ligament reconstructions: A systematic review and meta-analysis of 441 patients at a minimum 2 years' follow-up.
      ,
      • Harner C.D.
      • Janaushek M.A.
      • Kanamori A.
      • Yagi M.
      • Vogrin T.M.
      • Woo S.L.
      Biomechanical analysis of a double-bundle posterior cruciate ligament reconstruction.
      In their exceptional article “Dynamic Three-Dimensional Computed Tomography Mapping of Isometric Posterior Cruciate Ligament Attachment Sites on the Tibia and Femur: Single- Versus Double-Bundle Analysis,” Forsythe, Patel, Lansdown, Agarwalla, Kunze, Lu, Puzzitiello, Verma, Cole, LaPrade, Inoue, and Chahla
      • Forsythe B.
      • Patel B.
      • Lansdown D.A.
      • et al.
      Dynamic three-dimensional computed tomography mapping of isometric posterior cruciate ligament attachment sites on the tibia and femur: Single vs double bundle analysis.
      found that the position of the femoral tunnel and the angle of fixation contribute the most to the changes in PCL length. In double-bundle PCL reconstruction, the posteromedial bundle (PMB) had a high degree of isometry throughout the range of motion, whereas the anterolateral bundle was more anisometric. Forsythe et al. recommended fixation of the anterolateral bundle at 90° to avoid overconstraint of the graft. At the same time, single-bundle PCL reconstruction was associated with laxity at lower degrees of knee flexion.
      Given the fact that several biomechanical studies have shown that the native PMB is tight in knee extension, one can understand that the laxity seen in single-bundle reconstruction could be the result of the missing PMB. Indeed, with the knee in extension, the PMB serves to restrict posterior translation, whereas in knee flexion, the PMB restricts internal rotation.
      • Kennedy N.I.
      • Wijdicks C.A.
      • Goldsmith M.T.
      • et al.
      Kinematic analysis of the posterior cruciate ligament, part 1: The individual and collective function of the anterolateral and posteromedial bundles.
      ,
      • Chahla J.
      • Williams B.T.
      • LaPrade R.F.
      Posterior cruciate ligament.
      The distinct attachment sites of the 2 PCL bundles not only are suggestive of their synergistic and co-dominant role in knee biomechanics but also offer a technical advantage for safe tunnel placement when double-bundle reconstruction is attempted.
      The study by Forsythe et al.
      • Forsythe B.
      • Patel B.
      • Lansdown D.A.
      • et al.
      Dynamic three-dimensional computed tomography mapping of isometric posterior cruciate ligament attachment sites on the tibia and femur: Single vs double bundle analysis.
      adds to the series of recent work that has attempted to recognize important aspects in PCL reconstruction, such as the position of the femoral tunnel,
      • Jeong W.S.
      • Yoo Y.S.
      • Kim D.Y.
      • et al.
      An analysis of the posterior cruciate ligament isometric position using an in vivo 3-dimensional computed tomography-based knee joint model.
      the importance of fixation,
      • Zhang X.
      • Teng Y.
      • Li R.
      • et al.
      Proximal, distal, and combined fixation within the tibial tunnel in transtibial posterior cruciate ligament reconstruction: A time-zero biomechanical study in vitro.
      the timing from injury to reconstruction,
      • Paschos N.K.
      Editorial Commentary: Could time from injury to surgery affect decision making for posterior cruciate ligament reconstruction?.
      the graft bending angle,
      • Jang K.M.
      • Park S.C.
      • Lee D.H.
      Graft bending angle at the intra-articular femoral tunnel aperture after single-bundle posterior cruciate ligament reconstruction: Inside-out versus outside-in techniques.
      • Kim M.U.
      • Kim J.W.
      • Kim M.S.
      • Kim S.J.
      • Yoo O.S.
      • In Y.
      Variation in graft bending angle during range of motion in single-bundle posterior cruciate ligament reconstruction: A 3-dimensional computed tomography analysis of 2 techniques.
      • Weiss W.M.
      Editorial Commentary: Posterior cruciate ligament femoral techniques: The "critical corner" is just not as exciting as the "killer turn.
      the role of the tibial slope,
      • Bernhardson A.S.
      • Aman Z.S.
      • DePhillipo N.N.
      • et al.
      Tibial slope and its effect on graft force in posterior cruciate ligament reconstructions.
      the role of remnant tissue,
      • Jarvis D.L.
      • Waterman B.R.
      Editorial Commentary: Stump sparing or footprint exposing? Management of the tibial remnant during posterior cruciate ligament reconstruction.
      ,
      • Liu S.
      • Sun Y.
      • Chen T.
      • et al.
      Time from injury to surgery affects graft maturation following posterior cruciate ligament reconstruction with remnant preservation: A magnetic resonance imaging-based study.
      and the optimization of grafts.
      • Ansari A.S.
      • Dennis B.B.
      • Horner N.S.
      • et al.
      Influence of graft source on postoperative activity and joint laxity in posterior cruciate ligament reconstruction: A systematic review.
      ,
      • Martin R.K.
      • Levy B.A.
      Editorial Commentary: What is the best graft source for posterior cruciate ligament reconstruction?.
      The number of published articles about PCL injury showed an exponential increase in the past decade, reaching 76.9 and 82.9 articles published per year in the PubMed and Scopus databases, respectively (Fig 1). At the same time, several technical pearls have been presented, with the promise of a brighter future for our patients with PCL injury.
      • Chahla J.
      • Nitri M.
      • Civitarese D.
      • Dean C.S.
      • Moulton S.G.
      • LaPrade R.F.
      Anatomic double-bundle posterior cruciate ligament reconstruction.
      • Crawford M.D.
      • Kennedy M.I.
      • Bernholt D.L.
      • DePhillipo N.N.
      • LaPrade R.F.
      Combined posterior cruciate ligament and superficial medial collateral ligament knee reconstruction: Avoiding tunnel convergence.
      • Deal Jr., J.B.
      • Allen D.C.
      • Bottoni C.R.
      Anatomic double bundle posterior cruciate ligament reconstruction using an internal splint.
      • Ponzo N.
      • Del Castillo J.
      • Fregeiro J.
      • Kennedy M.I.
      • LaPrade R.F.
      Autograft anatomic, double-bundle posterior cruciate ligament reconstruction.
      • Rugg C.M.
      • Liu T.
      • Zhang A.L.
      Arthroscopic primary bundle-specific posterior cruciate ligament repair with transosseous fixation.
      Figure thumbnail gr1
      Fig 1Average number of posterior cruciate ligament (PCL) studies published per year by decade.
      It is extremely difficult to predict whether the improved biomechanics resulting from double-bundle reconstruction will translate into a measurable patient-reported outcome long term. However, discussion about optimizing the PCL reconstruction technique, as well as the recognition of the femoral attachment site position and the fixation angle as important factors, will inevitably lead to improvement in both single- and double-bundle PCL reconstructions.
      We have several historical examples in which underdogs were able to accomplish significant achievements against stronger opponents, such as David against Goliath and Leonidas against Xerxes. Traditionally, the critical error of the favored opponent is unawareness and overconfidence owing to greater size. With certain analogy, the smaller size of the PMB should not bias us to underestimate its importance in PCL reconstruction.

      Supplementary Data

      References

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