The loss of bone integrity that accompanies ACL injury persists after reconstructive surgery, even with aggressive rehabilitation. Improved interventions may help decrease patients’ risk for low bone mineral density and osteoarthrosis.
By John Nyland DPT, SCS, EdD, ATC, CSCS, FACSM
There is a growing emphasis in anterior cruciate ligament (ACL) reconstruction on more closely replicating the native ACL anatomy, thereby reestablishing a more functionally competent construct. By placing the ACL graft bundle or bundles within the normal anatomical footprint of the native ACL, it is widely believed that the double bundle function of the native ACL can be more closely replicated.1,2 Traditionally, arthroscopic ACL graft placement took on a more vertical orientation, with femoral tunnel positioning high on the lateral femoral intracondylar wall, approximating what has become referred to as “high noon” alignment closer to the center of the femoral condyle. Although this placement enabled good anterior-posterior knee stability as determined by instrumented laxity testing and historically good patient perceived functional capability, many patients that undergo ACL reconstruction have less than desirable results.2 Additionally, kinematic studies have revealed that traditional arthroscopic intra-articular ACL graft placement provides inadequate transverse plane rotational knee control.3,4
To improve patient outcomes, knee surgeons have presented arguments supporting the need to create a more anatomical ACL reconstruction, whether it be through placing a larger ACL graft more carefully within the “footprint” of the native ACL, or by creating individual graft bundles within that footprint.1,2 In the first instance, the philosophy is that by placing adequate graft volume in the anatomic footprint of the native ACL, the ligamentization and remodeling process will determine tissue functional differentiation. In the second instance, the philosophy is to specifically re-create each ACL bundle separately to better address both anterior-posterior (anteromedial bundle) and transverse plane rotational stability (posterolateral bundle) concerns. In the first instance, it is generally agreed that well-placed single femoral and tibial bone tunnels, with or without a bit of further tunnel notching, will suffice. In the second instance, it is believed that to more precisely replicate the functionality of individual ACL bundles, two separate tunnels at the femur and the tibia are needed. Both philosophies require greater graft tissue volume than the traditionally used bone-patellar tendon-bone (BPTB) autograft can provide, so knee surgeons are more commonly using longer and stronger soft tissue autografts (semitendinosis-gracilis tendons) or allografts (tibialis anterior or tibialis posterior tendons) for anatomic ACL reconstruction.5-7
A growing body of literature suggests that double bundle ACL reconstruction using two femoral and tibial tunnels can better control knee pivot shifting,8 however no evidence to date has shown that this surgical technique provides superior patient outcomes to ACL reconstruction using grafts of comparable size that are placed within the anatomic footprint of the native ACL. Additionally, to date no study has substantiated that any of these more anatomical approaches has resulted in a lower percentage of patients developing knee osteoarthrosis or in improved bone mineral density (BMD) in the distal femur and proximal tibia following ACL reconstruction and rehabilitation.1
Knee surgery and its associated procedures—related to tourniquet use,9 femoral or sciatic nerve block use,10 innovations in meniscal or articular cartilage preservation and surgical repair procedures, and current trends toward larger or a greater number of tunnels in the tibia and femur of the patient that has sustained an ACL injury1,2—may evoke a second trauma to the knee that warrants consideration, even when the involved tissue can be successfully reconstructed or repaired. In light of this, we propose that greater emphasis need be placed on how effectively osteoligamentous homeostasis can be restored following ACL reconstruction in addition to the restoration of ligamentous knee stability and more normal knee joint kinematics.
Bone in the surgical region of interest
Since trabecular cancellous bone has a higher turnover rate than cortical bone, it is a more sensitive indicator of bone remodeling during knee injury recovery.11 Kannus et al12 reported that at 10 to11 years post-ligament surgery, the critical factor regarding knee region bone mineral density (BMD) was how well patient function was restored. Ejerhed et al,11 however, noted that although most patients with an ACL injury had bone mineral area levels in the upper half of the normal age group population distribution prior to reconstruction, by two-years post-surgery most had levels that were more than two standard deviations below this distribution, despite improved function.
Most of the bone loss studies that included ACL reconstruction used BPTB autografts and interference screw fixation.11,13-19 Schmitz16 evaluated 24 patients at a mean of 2.5 years status-post ACL reconstruction using BPTB autografts, and reported that involved lower extremity BMD was 4.6% lower than the contralateral lower extremity at the femoral trochanteric region, 12.7% lower at the medial femoral condyle, 7.7% lower at the lateral femoral condyle, 6% lower at the distal femur, and 9.9% lower at the lateral patella. Each BMD reduction differed significantly from control group subjects. They also compared a group of 11 patients with a mean 8.5 year history of unilateral ACL deficiency to the same control group. Patients with unilateral ACL deficiency had BMD that was 1.1% lower than the contralateral lower extremity at the femoral trochanteric region, 3.5% lower at the medial femoral condyle, 5% lower at the lateral femoral condyle, 11.5% lower at the distal femur, and 10.3% lower at the lateral patella. Each of these BMD reductions also differed significantly from the control group. Their findings suggest that in addition to the index ACL injury, surgical reconstruction may also contribute to decreased BMD at the involved lower extremity.
In reporting their findings from a study of patients (age = 27.1 ± 8.8 years) following Achilles tendon allograft (n = 9) or BPTB autograft (n = 6) ACL reconstruction, Reiman et al18 reported significant BMD reductions at several hip sites at an average of 15 months post-surgery despite the fact that all patients had been treated by experienced physical therapists using established rehabilitation protocols.18 Although bone loss following ligamentous knee injury is known to be greater with prolonged immobilization, disuse, and decreased weightbearing, permanent bone loss appears to occur even with accelerated rehabilitation.11,14
Imaging and osseous homeostasis
Sequential bone scans have been successfully used to determine the restoration of knee joint osseous homeostasis, or a “happy knee,” following ACL reconstruction. Since osseous metabolism as determined by bone scans can be linked to knee kinematics, kinetics, lower extremity neuromuscular control, and patient perceived function, these scans are useful for identifying the loss of osseous homeostasis that might precede articular cartilage or meniscal lesions.20,21
Using technetium (Tc) 99m-labeled methylene diphosphonate (MDP) bone scans, Dye and Chew22 found widespread regions of increased osseous metabolic activity (suggesting abnormally increased bone turnover) in 85% (73 of 86) of patients with chronic symptomatic ACL deficiency, often with co-existing normal standard radiographs. Dorchak et al21 evaluated 50 consecutive patients with chronic ACL deficiency using Tc 99m-labeled MDP bone scans and correlated results with clinical examination, plain x-rays, and arthroscopic findings. They found that 92% of the scans revealed abnormally high osseous metabolic activity in the medial (88%), lateral (80%), and patellofemoral (66%) knee compartments. When the observed activity was scaled from 1 to 4 (1 = normal, 4 = marked activity) the highest abnormal activity levels occurred in the medial, followed by the lateral, and patellofemoral compartments.21 This finding confirmed that the sites most associated with of abnormally increased bone turnover were also the most seriously affected.
Dye and Chew23 reported the restoration of osseous homeostasis by 21 months in a 32-year-old male patient following ACL reconstruction. The ACL reconstruction procedure reported by Dye and Chew,23 however, made use of a traditional arthroscopic surgical approach and utilized a BPTB autograft, which is less commonly used for either anatomic or anatomic double bundle ACL reconstruction procedures. The BPTB autograft has a bone plug that fits and heals nicely into matched diameter bone tunnel tunnels, making it a more likely candidate to achieve earlier osseous homeostasis than a soft tissue tendon graft. However, concerns related to its limited graft size, poor ultimate mid-substance strength, and issues related to long-term graft harvest site complications (including extensor mechanism dysfunction, patella fracturing, patellofemoral joint dysfunction, patellar tendon shortening, and discomfort with kneeling) have decreased its popularity.5,24 Although studies have revealed that time-zero biomechanical properties are the same or better for soft tissue graft-bone tunnel fixation using a wide variety of fixation devices, it is also known that the soft tissue tendon graft-bone tunnel osteointegration process is slower, more complex, and more prone to stretch-injury than that associated with a bone plug healing in a bone tunnel.25-27 So, given the latest innovations in ACL reconstruction, knee surgeons have developed an approach with apparently better graft placement, but also with larger and/or more numerous tunnels that make greater use of soft tissue tendon grafts which undergo a different osteointegration process than conventional BPTB autografts.
Dorchak et al21 reported that standard radiographic findings correlated poorly with bone scan results unless moderate or severe structural changes were present. When bone scans were positive and articular cartilage surfaces appeared normal on arthroscopic inspection, Dorchak et al21 suggested that the bone scan was displaying subchondral changes not yet observable on the surface. Dorchak et al21 also reported that articular cartilage lesions observed during arthroscopy did not always manifest as abnormal bone scan results, suggesting that the osseous structures can sometimes return to homeostasis despite the presence of an abnormal articular cartilaginous surface. While bone scans can help evaluate osseous metabolic activity, magnetic resonance imaging (MRI) can better determine intra-tunnel tissue type and structure during graft-tunnel remodeling.28,29 In addition to evaluating structure and alignment, standard radiographs can be used to evaluate bone tunnel widening;30 however, they tend to underestimate true diameters.31 Multi-slice computed tomographic images can more accurately determine true trans-osseous tunnel boundaries.31 Sequential bone scans performed at key time periods before and after ACL reconstruction may help in monitoring patients deemed to be at particular risk for developing knee osteoarthrosis such as those with multiple ligament injuries, with concomitant meniscal injuries, or following revision surgery, particularly when deciding upon a safe time period to advance to more intense sport specific training.21,32 Establishing this safe time period is essential to preventing further knee injury, promoting increased BMD in the involved lower extremity, and decreasing the risk of developing knee osteoarthrosis.
Myers et al33 proposed that once patients who have undergone ACL reconstruction meet specific objective clinical criteria such as restoration of foundational lower extremity strength, and perceived function, involved and non-involved lower extremity vertical ground reaction force comparisons should be made during repeated, maximal effort single leg vertical hops of 10 seconds duration. Evidence of an involved lower extremity extensor power deficit compared to the non-involved lower extremity can identify patients in need of additional rehabilitation prior to being allowed to safely advance to more intense sport specific training.33 Biomechanical analysis of single lower extremity vertical ground reaction forces during locomotion, routine activities of daily living, and during functionally relevant athletic tasks (such as running directional changes and sudden stops) can provide valuable information about patient knee loading readiness and confidence or self efficacy34 following ACL reconstruction and rehabilitation. Premature return to unrestricted sport activities, before appropriate lower extremity loading biomechanics have been restored, increases the risk of knee re-injury and osteoarthrosis and also is likely to further decrease involved lower extremity BMD.
Rehabilitation programs should provide progressively increasing vertical impact acceleration loads and loading rates, concentric and eccentric lower extremity neuromuscular activation and control challenges (Figure 1), and non-habitual three-dimensional lower extremity loading stresses through both weightbearing and non-weightbearing exercises. Sequential vertical ground reaction force measurements during weightbearing exercises with involved and non-involved lower extremity comparisons can verify that patients have a foundation of lower extremity strength, power, and confidence to safely progress to more intense sport specific training. Therapeutic exercises that emphasize eccentric gluteus maximus, quadriceps femoris, hamstring, and gastrocnemius-soleus activation can improve lower extremity muscular shock absorption, prevent re-injury, enhance athletic performance, improve muscle-tendon injuries, increase BMD, and decrease fall risk.35 Gerber et al36 studied 40 patients at 15 weeks post-ACL reconstruction using either a BPTB or hamstring autograft, randomly assigning them to either a standard rehabilitation group or to a group that performed a 12 week duration progressive eccentric exercise program. At one year follow-up, compared to the standard rehabilitation group, quadriceps femoris (23.3 ± 14.1% vs. 13.4 ± 10.3%) and gluteus maximus (20.6 ± 12.9% vs. 11.6 ± 10.4%) MRI muscle volume displayed significantly greater increases at the involved lower extremity of the eccentric exercise group. Involved lower extremity quadriceps femoris isokinetic strength and single leg hop distance were also greater in the eccentric exercise group. Conceivably when the number of bone tunnels is increased to accommodate individual ACL bundle function using soft tissue tendon grafts, the rehabilitation clinician should place greater emphasis on maintaining appropriate osteoligamentous metabolic activity levels in the involved lower extremity during the rehabilitation progression, to improve graft-bone tunnel osteoligamentous integration and normal stress transfer across the knee joint.
Re-establishing enthesis function
Anatomical ACL graft placement enables knee surgeons to more closely match native ACL tibial and femoral “footprints.” However, we need to more objectively and quantitatively determine the effect of the intervention on restoring pre-injury lower extremity BMD. We also need to determine how effectively soft tissue tendon grafts placed in bone tunnels or smaller sockets can simulate three-dimensional ACL enthesis function, attachment site stress concentrations, and femur-tibia load transfer. Effective stress transfer depends on accurate graft placement, tunnel region trabecular cancellous bone architecture, the mechanical properties of both tissues, and the integrity of early fixation.25,37-40 The enthesis of a ligament or tendon in one knee region generally overlaps with another, adding to composite anchorage stability and to osteogenic stimulation.37,39 Therefore concomitant ACL and medial collateral ligament injury may impair osteogenic stimulation just as it impairs ligamentous stability. Ideally, the reconstructed ACL entheses remodel into three-dimensional footprints with “roots like a tree,”39 closely simulating native ligament-bone stress transfer and re-establishing the osteogenic stimulation that improves bone health.38-40 To ensure that optimal BMD responses can occur with appropriate exercise stimulation, patients should be counseled about the importance of ingesting appropriate levels of dietary calcium, phosphorous, and vitamin D.41,42
Involved lower extremity bone integrity is decreased following ACL injury, and existing evidence suggests that it currently is not restored after ACL reconstruction even with accelerated rehabilitation. Anatomically positioned and securely fixed soft tissue tendon grafts should more effectively re-establish both three-dimensional knee ligamentous stability and enthesis stress transfer between the femur and tibia. This should facilitate improved lower extremity BMD and better restore osteoligamentous homeostasis. Diagnostic imaging, including bone scans, MRI, and standard radiographs, performed at key time points before and after ACL reconstruction can objectively and quantitatively help determine metabolic and structural tissue healing, remodeling, and osteoligamentous homeostasis.
We need to more objectively and quantitatively determine the effect of our interventions on restoring pre-injury lower extremity BMD. We also need to determine how effectively soft tissue tendon grafts secured in bone tunnels or smaller sockets can simulate three-dimensional ACL enthesis function, attachment site stress concentrations, and femur-tibia load transfer. As ACL reconstruction with extra bone tunnels and greater use of soft tissue tendon grafts become more common, we need to become more objective and quantitative in how we evaluate our ability to increase involved lower extremity BMD (particularly at the enthesis regions) in addition to restoring ligamentous stability and meeting current International Knee Documentation Committee standards. We also need to better verify that surgical and rehabilitative interventions reestablish comparable involved and non-involved lower extremity loading biomechanics during relevant sport movements prior to sport specific training and return to play decision making, that patient confidence and self efficacy are improved, that the onset and severity of knee osteoarthrosis are reduced, and ultimately that ACL reconstruction revision rates decrease.
John Nyland DPT, SCS, EdD, ATC, CSCS, FACSM, is associate professor in the division of sports medicine, department of orthopaedic surgery, and advisory dean in the school of medicine at the University of Louisville in Louisville, KY.
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