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Editorial Commentary: The Hip Bone's Connected to the Knee Bone, but Correlation Does Not Equal Causation—The Association of Hip Motion, Femoroacetabular Impingement, and Anterior Cruciate Ligament Injury

      Abstract

      Patients with anterior cruciate ligament (ACL) injury had significantly less hip rotational motion (internal rotation [IR] and sum of IR and external rotation) than control subjects without ACL tears. For each hip IR increase of 10°, the odds of sustaining an ACL rupture decreased by a factor of 0.419. Although this investigation does not prove (causation) that loss of hip rotational motion causes an ACL tear, it does continue to complement the growing and evolving literature base showing an upstream or downstream association (correlation) of decreased hip motion on adjacent structures.
      “You tore your ACL.” This phrase immediately strikes fear into the hearts of athletes across the world. In the United States alone, over 250,000 anterior cruciate ligament (ACL) injuries occur annually.
      • Mall N.A.
      • Chalmers P.N.
      • Moric M.
      • et al.
      Incidence and trends of anterior cruciate ligament reconstruction in the United States.
      This number is growing at an alarming rate, despite the proliferation of ACL injury prevention programs.
      • Leathers M.P.
      • Merz A.
      • Wong J.
      • Scott T.
      • Wang J.C.
      • Hame S.L.
      Trends and demographics in anterior cruciate ligament reconstruction in the United States.
      Because most ACL ruptures are noncontact injuries, correction of modifiable neuromuscular and biomechanical pathology remains an integral component of ACL injury prevention.
      • Hewett T.E.
      • Ford K.R.
      • Xu Y.Y.
      • Khoury J.
      • Myer G.D.
      Utilization of ACL injury biomechanical and neuromuscular risk profile analysis to determine the effectiveness of neuromuscular training.
      The effectiveness of these programs is a subject of great controversy and debate, with proponents citing risk reduction based on laboratory investigations and opponents citing the impracticality and challenge of implementation. The key word that cannot be emphasized enough is “modifiable.” That is where the hip comes in.
      The relation between hip motion and ACL injury is the theme of the article “Restricted Hip Rotation Is Correlated With an Increased Risk for Anterior Cruciate Ligament Injury” by VandenBerg, Crawford, Sibilsky Enselman, Robbins, Wojtys, and Bedi.
      • VandenBerg C.
      • Crawford E.A.
      • Sibilsky Enselman E.
      • Robbins C.B.
      • Wojtys E.M.
      • Bedi A.
      Restricted hip rotation is correlated with an increased risk for anterior cruciate ligament injury.
      The authors performed a prospective case-control investigation of 25 ACL-injured male and female patients (aged 12-40 years) and 25 age- and gender-matched controls with the explicit intent of determining if a correlation exists between hip internal rotation (IR) and ACL tear. This article does an excellent job of illustrating that “correlation does not equal causation.” Stress transfer from the hip to other upstream (proximal)
      • Gebhart J.J.
      • Weinberg D.S.
      • Conry K.T.
      • Morris W.Z.
      • Sasala L.M.
      • Liu R.W.
      Hip-spine syndrome: Is there an association between markers for cam deformity and osteoarthritis of the lumbar spine?.
      • Birmingham P.M.
      • Kelly B.T.
      • Jacobs R.
      • McGrady L.
      • Wang M.
      The effect of dynamic femoroacetabular impingement on pubic symphysis motion: A cadaveric study.
      • Saito M.
      • Kenmoku T.
      • Kameyama K.
      • et al.
      Relationship between tightness of the hip joint and elbow pain in adolescent baseball players.
      or downstream (distal)
      • Ellera Gomes J.L.
      • Palma H.M.
      • Ruthner R.
      Influence of hip restriction on noncontact ACL rerupture.
      • Gomes J.L.
      • de Castro J.V.
      • Becker R.
      Decreased hip range of motion and noncontact injuries of the anterior cruciate ligament.
      • Lopes Jr., O.V.
      • Gomes J.L.
      • de Freitas Spinelli L.
      Range of motion and radiographic analysis of the hip in patients with contact and non-contact anterior cruciate ligament injury.
      structures is increasingly being recognized as a distinct entity that may manifest clinically through pain and/or injury. Notwithstanding, VandenBerg et al. do not attempt to prove the latter. They simply show an association between decreased hip motion and increased odds of ACL injuries. However, this is not a new concept, as other authors have proffered similar findings.
      • Ellera Gomes J.L.
      • Palma H.M.
      • Ruthner R.
      Influence of hip restriction on noncontact ACL rerupture.
      • Gomes J.L.
      • de Castro J.V.
      • Becker R.
      Decreased hip range of motion and noncontact injuries of the anterior cruciate ligament.
      • Lopes Jr., O.V.
      • Gomes J.L.
      • de Freitas Spinelli L.
      Range of motion and radiographic analysis of the hip in patients with contact and non-contact anterior cruciate ligament injury.
      • Tainaka K.
      • Takizawa T.
      • Kobayashi H.
      • Umimura M.
      Limited hip rotation and non-contact anterior cruciate ligament injury: A case-control study.
      Furthermore, the radiographic indicators of cam and pincer femoroacetabular impingement (FAI) have been associated with ACL tears.
      • Philippon M.
      • Dewing C.
      • Briggs K.
      • Steadman J.R.
      Decreased femoral head-neck offset: A possible risk factor for ACL injury.
      The exact cause-effect role of the hip in the pathogenesis of ACL injury is controversial, highly timely, certainly worthy of investigation, and in need of an answer.
      So what are we to do with this relation? Is hip motion modifiable? The implications, both nonsurgically and surgically, are significant. In a separate Level IV evidence investigation from the National Football League Combine, the authors created an in silico pivot landing biomechanical model, showing that FAI may be associated with ACL injury through resistance to hip (femoral) IR.
      • Bedi A.
      • Warren R.F.
      • Wojtys E.M.
      • et al.
      Restriction in hip internal rotation is associated with an increased risk of ACL injury.
      Less IR at the hip is compensated for at the knee through increased tibial IR and a subsequent increase in ACL strain—hence, ACL injury risk.
      • Beaulieu M.L.
      • Oh Y.K.
      • Bedi A.
      • Ashton-Miller J.A.
      • Wojtys E.M.
      Does limited internal femoral rotation increase peak anterior cruciate ligament strain during a simulated pivot landing?.
      • Oh Y.K.
      • Kreinbrink J.L.
      • Wojtys E.M.
      • Ashton-Miller J.A.
      Effect of axial tibial torque direction on ACL relative strain and strain rate in an in vitro simulated pivot landing.
      Thus, if a lack of hip motion, “especially IR,” is identified, “appropriate counseling…focused therapy and appropriate training modifications may improve hip function and help prevent ACL injuries.”
      • VandenBerg C.
      • Crawford E.A.
      • Sibilsky Enselman E.
      • Robbins C.B.
      • Wojtys E.M.
      • Bedi A.
      Restricted hip rotation is correlated with an increased risk for anterior cruciate ligament injury.
      Still unknown though is the ability to attain significant nonsurgical improvement in motion by simple stretching.
      • Wall P.D.
      • Dickenson E.J.
      • Robinson D.
      • et al.
      Personalised hip therapy: Development of a non-operative protocol to treat femoroacetabular impingement syndrome in the FASHIoN randomised controlled trial.
      However, stretching is a common component of a standard athletic training physiotherapy program. For example, FIFA 11+ is a highly effective, international injury prevention program for soccer players, composed of 15 exercises in 3 parts, in which stretching and core strength form the foundation of the program.
      • Bizzini M.
      • Dvorak J.
      FIFA 11+: An effective programme to prevent football injuries in various player groups worldwide—A narrative review.
      However, according to the recent Warwick Agreement, FAI syndrome is a motion-related (flexion and rotation) clinical disorder of the hip with a triad of symptoms, clinical signs, and imaging findings, representing symptomatic premature contact between the proximal femur and acetabulum.
      • Griffin D.R.
      • Dickenson E.J.
      • O'Donnell J.
      • et al.
      The Warwick Agreement on femoroacetabular impingement syndrome (FAI syndrome): An international consensus statement.
      Thus, overly aggressive stretching and attempts to attain more motion could exacerbate symptoms. Future research should address this delicate balance of improved motion without creating or worsening symptoms. Until proved otherwise, programs such as FIFA 11+ and Warwick's Personalized Hip Therapy,
      • Wall P.D.
      • Dickenson E.J.
      • Robinson D.
      • et al.
      Personalised hip therapy: Development of a non-operative protocol to treat femoroacetabular impingement syndrome in the FASHIoN randomised controlled trial.
      under guidance and supervision with proper technique, may be used with our own patients and athletes to optimize performance and reduce injury risk.
      Operatively, hip ROM, “especially IR,” significantly increases after comprehensive arthroscopic correction of FAI.
      • Bedi A.
      • Dolan M.
      • Hetsroni I.
      • et al.
      Surgical treatment of femoroacetabular impingement improves hip kinematics: A computer-assisted model.
      • Kelly B.T.
      • Bedi A.
      • Robertson C.M.
      • Dela Torre K.
      • Giveans M.R.
      • Larson C.M.
      Alterations in internal rotation and alpha angles are associated with arthroscopic cam decompression in the hip.
      • Matsuda D.K.
      • Schnieder C.P.
      • Sehgal B.
      The critical corner of cam femoroacetabular impingement: Clinical support of an emerging concept.
      Certainly, this does not mean prophylactic FAI correction for asymptomatic patients to reduce ACL injury risk (“treat patients, not x-rays”). But it does add another factor to the multifaceted biomechanical equation explaining failure of a previous ACL reconstruction. In the setting of “associated symptomatic hip impingement,” it is “an additional reason for treatment.”
      • VandenBerg C.
      • Crawford E.A.
      • Sibilsky Enselman E.
      • Robbins C.B.
      • Wojtys E.M.
      • Bedi A.
      Restricted hip rotation is correlated with an increased risk for anterior cruciate ligament injury.
      On the basis of this study, together with the rapidly growing base of evidence, clinicians and researchers evaluating and treating patients with ACL injuries must recognize that a relationship exists between hip motion and ACL tears, enhance the understanding of this relationship, and improve not only our treatment of these patients but better yet the prevention of such injuries.

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