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Anterior cruciate ligament injuries of the knee

Anatomy

The anterior cruciate ligament's (ACL) name is derived from its anterior insertion on the tibial plateau and the fact that it "crosses" the posterior cruciate ligament within the intercondylar notch ( Latin: crux, cruc-, cross). The femoral attachment is at the posteromedial aspect of the lateral femoral condyle. The tibial attachment is larger and more stable than its attachment to the femur and is located medial to the insertion site of the anterior horn of the lateral meniscus, 15mm posterior to the anterior border of the tibial articular surface [1] . The ACL itself consists of two bundles named after their tibial insertion sites, the anteromedial (AM) bundle and the posterolateral (PL) bundle . The AM bundle tightens when the knee is in flexion. The PL bundle tightens when the knee is in extension and also tightens during internal and external rotation of the tibia. The primary blood supply is the middle geniculate artery which enters the ligament near its femoral attachment after entering the posterior capsule. There is collateral blood supply from the medial and lateral geniculate arteries. The innervation of the ACL is from the tibial nerve via the posterior articular branch, which is primarily vasomotor in function. Mechanoreceptors and possibly pain fibers are present as well. The typical size of the ligament is anywhere from 30mm to 37mm in length and an averages 40mm squared in cross section at midsubstance.

Biomechanics

Function

  • Primary function: provides approximately 85% of the resistance to anterior tibial translation
  • Secondary functions include restraint to varus/valgus forces and it provides rotational stability through the posterolateral bundle

Pathogenesis

  • Most injuries are secondary to low energy mechanisms.
    • Two thirds of the injuries are the result of a noncontact injury (hyperextension or pivoting). This is the type of injury often seen in athletes of "cutting" sports such as soccer, skiing, and football.
    • Direct contact injuries often induce a hyperextension or valgus stress on the knee that leads to a cruciate injury.
  • High energy mechanisms, such as motor vehicle accidents are often associated with concurrent injuries.
  • Compared to males, females have a decreased protective role of dynamic knee stabilizers, decreased ability to resist anterior tibial translation with muscle co-contraction, a smaller ACL, and decreased intercondylar notch width index [2]. Additionally, females may be at increased risk of ACL injury during different phases of the menstrual cycle . Female basketball players have up to a 3.5 incidence ratio of ACL tears compared to males. For soccer players the ratio is 2.67, lacrosse players 1.18, and for Alpine skiing 1.0  .

Natural History

Damage to extraarticular ligaments results in a local hematoma which leads to the formation fibrinogen mesh, accumulation of inflammatory cells, granulation tissue, and ultimately fibrous tissue that restores function to the ligament. This is partially made possible by the surrounding soft tissues that contain the hematoma, allowing this cascade of events to occur. The cruciates, however, are only surrounded by a thin synovial membrane. This membrane may be preserved in partial rupture, thereby providing containment of the hematoma and allowing some degree of fibrous tissue formation. In complete tears this membrane ruptures, allowing the hematoma to dissipate throughout the joint and prohibiting the formation of fibrous scar tissue. Therefore, in the setting of a complete tear, a functional ACL can only be restored by operative intervention.

One prospective randomized study comparing nonoperative management with ACL repair or reconstruction showed no difference in rates of radiographic osteoarthritis at 15 years followup. However, 1/3 of patients in the nonoperative group required surgical reconstruction due to persistent instability. The status of the menisci was the strongest predictor for the development of osteoarthritis. If a meniscectomy was performed, 2/3 of the patients developed radiographic osteoarthritis regardless of the ACL treatment group .

However, it has been shown that ACL reconstruction decreases the risk of future meniscal injury when compared to nonoperative management .

Patient History and Physical Findings

  • History -  Two thirds of the injuries are the result of a non contact injury (deceleration or pivoting) and are often associated with a "pop" and swelling, which is typically seen within 4-12 hours fo the injury. (Other knee injuries associated with a hemarthrosis include posterior cruciate tear, peripheral meniscus tear, osteochondral fracture, capsular injury, and patellar dislocation. Direct contact injuries often induce a hyperextension or valgus stress on the knee that leads to a cruciate injury. Other questions include ability to bear weight? Did the patient continue play? Symptoms of instability? Other factors to consider include pre-injury activity level, job activities, and future plans, as this information will aid in decision making.
  • Examination - Clinical examination is at least as accurate as MRI for the diagnosis of ACL tear in the hands of a skilled examiner . Examination immediately after the injury is typically more accurate than after the onset of swelling, pain, and subsequent guarding.
  • Observation: malalignment may be suggestive of a fracture. Swelling typically appears by 4 hours.
  • Palpation: effusion? concurrent injuries?
  • Range of motion (active and passive)
  • Stability testing: AP, varus, valgus, rotational.
    • Lachman Test: is more sensitive and used more often that he anterior drawer test for testing anteroposterior stability.
    • Pivot Shift Test: findings have been reported to have high correlation with functional outcomes after ACL reconstruction .
    • Arthrometers: KT-1000 - Quantifies only anteroposterior stabiltiy. Does not quantify rotational stability.

Imaging and other Diagnostic Studies

X-ray: Useful in the diagnosis of associated injuries

  • Segond sign - A lateral tibial plateau avulsion is highly suggestive of an ACL tear (present in approximately 9% of ACL tears) .
  • Tibial tubercle avulsion - often seen in younger patients and those with poor bone quality

MRI: Typically the diagnosis of ACL tear can be made on the basis of history and physical alone. Diagnosis can be confirmed with MRI. Reported sensitivity ranges from 90-95% and specificity from 95-100% . Damage to ACL seen on one slice may be indicative of a complete, partial or intra-substance tear. Images must be reviewed in all planes to confirm diagnosis. Positive signs on MRI suggestive for complete ACL tear include :

  • Direct signs:
    • A poorly demonstrated or lack of ACL on all sagittal images
    • Well-defined mass in the intercondylar notch with high signal intensity on T2-weighted images
    • ACL disruption on axial and sagittal images and failure of the ACL fascicles to parallel Blumensaat's line
    • Fluid signal in the proximal ACL (common in ruptures in skiers)
  • Indirect signs:
    • A kissing contusion, (ie a bone bruise on the posterior-lateral tibia and central lateral femur in the terminal sulcus), can occur only with pathological translation and thus suggests an ACL tear
    • Anterior Drawer Sign: Present when the posterior tibial border is more than 5mm anterior to a vertical line drawn tangential to the posterior femoral condyle
    • Buckled PCL: occurs when  any portion of the PCL is concave posteriorly
    • Posterior PCL line, coronal PCL sign, posterior femoral line, Buckling of the patella tendon

Diagnosis of Partial ACL Tears

The diagnosis of a partial tear is difficult and requires the use of a combination of history, physical exam, MRI and possibly arthrometer testing. MRI findings suggestive of a partial tear include disruption of either the AM or PL bundles, focal edema or focal thickening of the ACL, diffuse thickening of the ACL with some detectable fibrillar pattern, and lack of a bone bruise [1]. The addition of oblique sagittal and coronal planes to MRI imaging has been shown to increase accuracy [1].

Associated Injuries

Medial collateral ligament injuries of the knee

In the setting of combined ACL/MCL injuries, lateral meniscus tears are more commonly seen than medial meniscus injuries. A common tear pattern entails a radial split tear of the middle horn and vertical tear of the posterior horn.

Medial meniscus tears are more commonly seen with chronic ACL injuries.

Differential Diagnosis

Hemarthrosis in the setting of normal x-rays can be due to an ACL tear, a patellar dislocation, a peripheral meniscal tear, or an osteochondral fracture.

Treatment of Complete ACL Tears

Nonoperative treatment:  Patients should be counseled that nonoperative treatment will place them at risk of continued instability, and progressive meniscal and chondral degeneration. However, nonoperative treatment may be the best option in older, low demand patients. Treatment primarily involves aggressive quadriceps and hamstring strengthening.

Operative treatment: There is no upper age limit for consideration of operative treatment. In general, surgical reconstruction is recommended for young patient, those with high activity levels (particularly those involved in sports requiring cutting an pivoting) and those with greatly increased laxity. Reconstruction is typically performed once swelling, range of motion, pain level and quadriceps control have been optimized.

  • Graft Selection -  A 2010 systematic review of Oxford Level I & II studies reported no difference between autograft and allograft reconstructions . Likewise, a 2009 prospective randomized study reported no difference between BPTB and hamstrings tendon autograft reconstructions at minimum 2 years followup . However, there are many factors to consider when choosing a graft type. 
  • Allograft reconstruction: Allografts available include bone-patellar tendon-bone, achilles tendon, quadriceps tendon, tibialis anterior, tibialis posterior, hamstring tendon grafts, and fascia lata. Advantages of allografts include no donor site morbidity and less functional limitations in the short-term . Allografts are typically preferred for the lower demand or older patient. Risks of allograft use include disease transmission and immunogenicity. Disease transmission has decreased significantly with the use of polymerase chain reaction (PCR) testing. Click here for a summary on the safety of tissue transplants published by the American Association of Tissue Banks (AATB).  Currently, the AATB requires that donor tissue be screened for HIV, hepatitis B and C, syphilis, human T-cell lymphotropic virus. Additionally donor blood and tissue cultures are performed to screen for bacterial infection 1. Cryopreservation of the allografts during preparation kills donor cells and may denature cell surface histocompatibility antigens. As a result, the host immune response is limited .
  • Autograft reconstruction: Most commonly used grafts include either a "bone-patellar tendon-bone" (BPTB) autograft, quadrupled hamstring autograft consisting of the distal semitendonosis and gracillis tendons, and quadriceps tendon autograft. Competitive football players are typically reconstructed using bone-patellar tendon-bone autograft. Competitive basketball players are typically reconstructed using hamstrings autograft in order to avoid future patellar tendon pathology in light of the jumping requirements of this sport . Donor site morbidity is a factor to consider when deciding on graft type. Donor site pain, local numbness, patellar fractures, patellar tendon ruputres (rare), and patellar tendonitis may be seen after BPTB autograft reconstructions. The presence of increased anterior knee pain after BPTB autograft reconstruction compared to other methods of reconstruction is controversial. Hamstring autograft harvest may damage the infrapatellar branch of the saphenous nerve. 
  • Single bundle vs. double bundle - Neither technique has been shown to be clinically superior to the other as of yet, however research is ongoing. Numerous studies have shown improved AP and rotational stability as well as improved femoral-tibial contact area suggesting that double bundle ACL reconstruction more closely restores knee kinematics when compared to single bundle reconstruction  [1].
  • Graft tension - The level of intraoperative graft tensioning and knee position at which this is performed is controversial, as is preconditioning of the graft. A recent systematic review of randomized controlled trials concluded that initial graft tension does not affect clinical outcomes, regardless of graft type .
  • Graft fixation -
    • Techniques
      • Direct: interference screws, staples, spiked washers
      • Indirect: cross-pin, screw and post, EndoButton
    • BPTB grafts most commonly employ interference screws on both the femoral and tibial side. Soft tissue grafts may be secured by a variety of methods, with no consensus as to what is the best. As a rule, fixation on the tibial side is more difficult owing to the softer bone of the plateau, parallel tibial tunnels, and a interference screw insertion direction that is counter to the direction of graft tension [1].
    • A recent systematic review of randomized controlled trials concluded that bioabsorbable and titanium screws produce similar clinical results when used for hamstring and patellar tendon reconstructions. However, bioabsorbable screws interfere less with future MRI and surgery and theoretically have a lower risk of late hematogenous infection. This review also concluded that that cross pin fixation and interference screws are comparable means of fixating a hamstring tendon graft .

Indications and contraindications

Treatment of ACL tears in the skeletally immature:

Biology of cruciate ligament reconstruction

Allografts heal in the same manner as autografts but at a much slower rate. Early tendon to bone healing involves the formation of fibrovascular scar tissue at the graft tunnel interface followed by the formation of Sharpey's fibers composed of type III collagen. Ultimately bone grows into the interface tissue and incorporates the outer portion of the graft. Soft tissue grafts typically heal to bone in 8-12 weeks. The bone to bone healing process first involve osteonecrosis of the bone plug followed by the incorporation of surrounding cancellous bone. Bone to bone healing typically occurs within 6 weeks. Intraarticularly the autograft tendon initially undergoes a period of avascular necrosis. Graft strength decreases greatly from approximately 3 weeks until 6 months after surgery. Ultimately, autograft and allograft soft tissue grafts provide scaffold for re-population by host synovial cells . The final phase of graft incorporation involves collagen maturation. Revascularization stems from the infrapatellar fat pat and the posterior synovial tissues . The graft never fully resembles the structure of a native ACL or that of the original tendon [1].

Pearls and Pitfalls

Tunnel malposition is one of the most common indications for revision of a single-bundle ACL. Tunnel position that is too vertical may result in insufficient rotational stability. The current trend is to place the femoral tunnel at the 10 o'clock/2 o'clock position .

Postoperative Care

Cryotherapy - A meta-analysis has reported crytherapy to decrease postoperative pain, however it did not have an effect on range of motion or wound drainage. The study concluded that in light of the ease of use, low cost and low side effect profile, and high patient satisfaction, postoperative use of cryotherapy is justified .

Bracing - Rehabilitation braces are used during the postoperative period, while functional braces are used when the patient returns to play. At least two studies have reported that rehabilitation brace locked in extension or hyperextension during the early postoperative period has been shown to prevent loss of extension . However, a systematic review of randomized controlled trials published in 2009 concluded that postoperative bracing neither affects clinical outcome nor reduces the risk of subsequent intra-articular injury . The benefits of functional bracing is controversial [1].

Weight-bearing - Immediate weight-bearing has been reported to be associated with lower incidence of anterior knee pain and no difference in knee extension range of motion , VMO function, and AP knee laxity when compared to 2 weeks of delayed weight-bearing in 49  prospectively randomized patients undergoing BPTB autograft reconstruction .

Physical Therapy - In general closed kinetic chain (CKC) exercises are recommend, as they place less stress on the ACL graft compared to open kinetic chain (OKC) exercises. A review of the literature published in 2009 has concluded that CKC exercised produce less pain and risk of increased laxity and better subjective outcomes in patients who have undergone BPTB reconstructions. This comparison has not been made in patients who have undergone HS reconstructions . However, numerous prospective randomized clinical studies have failed to show a significant difference between closed and open chain exercises in graft healing and functional outcomes [1]. Attention should be given to both quadriceps and hamstrings strengthening. Eccentric exercises may result in increased quadriceps and gluteus maximus volume and function at one year postoperatively . Electrical stimulation and biofeedback have both been shown to improve recovery of quadriceps function postoperatively as well [1].

Rehab protocol - In general rehab protocols focus on early range of motion, immediate weight-bearing as tolerated, and return to sports at 6-9 months. Decision making for return to play should be made on a case by case basis. Criteria for return to play status includes range of motion, muscles strength and balance, static stability, and dynamic stability as measured by functional testing [1].

Outcome

A knee with an isolated ACL injury that has been reconstructed has a very low risk (0-13%) of developing premature osteoarthritis more that 10 years after the injury . Associated  meniscal injuries increase the risk to 21-48% risk for the development of premature osteoarthritis .  One study has reported that history of meniscectomy but not ACL reconstruction is associated with a shortened expected career in the NFL . For more info on outcomes in NBA and NFL athletes, see the following papers: .

Complications

Arthrofibrosis is the most common postoperative complication.

Persistent instability

Septic arthritis: Incidence of 0.3-1.7% .

Patellar fractures, patellar tendon ruputres, and patellar tendonitis may be seen after BPTB autograft reconstructions

Disease transmission from allograft tissue is extremely rare

Selected References

[1] DeLee, Jesse C., David Drez Jr., and Mark D. Miller, eds. DeLee & Drez's orthopaedic sports medicine principles and practice. 3rd ed. Vol. 2. Philadelphia: Saunders/Elsevier, 2010.

[2] Garrick, J. G. (Ed.). (2004). Orthopaedic Knowledge Update: Sports Medicine (3rd ed.). Rosemont, IL: American Academy of Orthopaedic Surgeons.

Internet Resources

http://www.vumedi.com/profile/FreddieFu

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