Technique: Complete Transphyseal Hamstring Autograft

Nathan A. Mall

George A. Paletta Jr.

INTRODUCTION

The number of anterior cruciate ligament (ACL) reconstructions performed in the United States has been increasing, with the most significant rises seen in patients younger than the age of 20 years.¹ Long-term data demonstrates a clear advantage with ACL reconstruction over conservative treatment in terms of risk of surgery for meniscal or cartilage damage, Tegner activity scores, and rotational instability in the knee (positive pivot shift).² This data has caused a paradigm shift in which most orthopedic surgeons would recommend surgical intervention for ACL tears even in skeletally immature patients. Previously, the risk of damage to the growth plate was thought to override any risk resulting from an unstable knee and patients were typically given an ACL brace and restricted from sports. Currently, the risk of the irreversible meniscal or cartilage damage is thought to outweigh the small risk of growth plate injury and possible resultant malalignment or limb length discrepancy.³ Several techniques have been described for ACL reconstruction in the still skeletally immature athlete. Some authors have proposed algorithms based on skeletal age and projected growth remaining to assist surgeons in determining the proper reconstruction procedure for each individual patient.⁴ Each proposed technique has its unique associated technical challenges and potential complications. Transphyseal reconstruction can provide an anatomic type reconstruction and avoid some of the potential complications related to other techniques with minimal risk to the growth plate. This chapter outlines the technical pearls and data supporting transphyseal hamstring autograft reconstruction in the skeletally immature ACL-injured patient.

GRAFT SELECTION

Bone Plug versus Soft Tissue

Bone-patellar tendon-bone (BTB) autografts have been considered as the gold standard ACL graft by many surgeons. However, the characteristics and biomechanical profile of the BTB autograft is not necessarily the closest to that of the native ACL.⁵ Some authors have raised concern over placing a bone plug, screw, or both across the physis, believing that this may increase the risk of physeal closure.⁶ Two studies, one using periosteum and the other using bone plugs, performed BTB autografts in children with open physes. Neither study reported significant growth disturbance with this technique, although the bone plug or periosteum was placed proximal to growth plate with suspensory cortical fixation. In using these techniques effectively, a soft tissue graft was used across the physis. Although no large studies have evaluated the effect of placing a bone plug across a growing physis, it is generally accepted that this should be avoided, as the risk of formation of a physeal bar would be high. Additionally, harvest of a BTB graft in the skeletally immature would result in violation of the tibial tubercle apophysis with risk of premature closure of the anterior aspect of the proximal tibial physis and the development of a recurvatum deformity. Thus, hamstring grafts have become the gold standard in the skeletally immature patient.⁶,⁷,⁸,⁹

Autograft versus Allograft

The literature documents clear differences in revision rates for primary ACLs done with allograft and autograft in young, athletic patients. In a study evaluating 120 cadets entering the U.S. Military Academy, the authors found a 7.7-fold increase in failure rate following ACL reconstruction using allograft as compared with autograft.¹⁰ Ellis et al.¹¹ found a 15-fold increase in revisions in patients who were skeletally mature but whose age at time of surgery was younger than 18 years when BTB allograft was used as compared with BTB autograft. Another study evaluated risk factors for revision ACL reconstruction and again demonstrated increased graft failure in the young patient (odds ratio, 0.40 for 10-year increase in age) and with allograft tissue (odds ratio, 3.97) used for the ACL reconstruction.¹² Inacio et al.¹³ evaluated graft choice and fellowship status as well as case volume, finding that non-fellowship-trained surgeons and low-volume surgeons were more likely to choose allografts. Considering these factors, the authors strongly prefer autograft tissue for ACL reconstruction in the skeletally immature.

Graft Size

Magnussen et al.¹⁴ found that a graft of 8-mm diameter or less produced an odds ratio of 2.20 for failure and subsequent revision. Interestingly, however, the odds ratio for revision of patients younger than the age of 20 years was 18.97. When closely evaluating this study, 16 of the 18 patients requiring revision were both younger than age 20 years and grafts are smaller than 8 mm, making it impossible to differentiate if graft size, age, or both are the true factors in the higher risk of revision. Similarly, a study by the Multicenter Orthopaedic Outcomes Network (MOON) group noted that the only revisions occurred in patients with grafts less than 8 mm; however, 13 of the 14 revisions occurred in patients aged 18 years or younger. Both these studies suggest there may be a higher failure and revision rate in younger patients. The relationship between graft size and failure may not be fully elucidated, as patient age seems to be a confounding factor. However, in the MOON group study, of the 85 patients aged 18 years and younger, only 14 had grafts measuring 8 mm or larger and none of those required a revision.¹⁵

In fact, grafts too large may also be a problem, leading to graft impingement against the intercondylar notch, lateral wall, or posterior cruciate ligament (PCL).¹⁶,¹⁷,¹⁸ Ichiba et al.¹⁸ found that the ACL insertion on the tibia correlated with height and weight of the patient. Both the height and area of the lateral wall of the intercondylar notch have been shown to correlate with the native ACL footprint size on both the femur and the tibia. Another study evaluated the size of various grafts compared with the native footprint, demonstrating that the semitendinosus and gracilis combined hamstring graft may be too big compared to the native ACL. However, this study did not evaluate patient size or bony dimensions in relation to native ACL size or graft size. Thus, perhaps, size of the graft should be based on normative values related to the size of patient. Once these values are noted, augmentation of grafts with cadaveric or suture material may be used for patients with abnormally small hamstring or patellar tendons.

TRANSPHYSEAL DATA

Studies of Effects of Physeal Involvement

Magnetic resonance imaging (MRI) studies of transphyseal techniques demonstrated a mean destruction of the femoral physis of 5.4% using a 9-mm drill. The maximum amount of destruction was 8.8% of the femoral physis, which depended on the obliquity of the tunnel.¹⁹ At the tibial insertion, the mean was 3.8% of the total volume of the physis, with a maximum of 6.6%, again with 9-mm tunnels. In an MRI study of patients undergoing transphyseal reconstruction, the authors found a bone tunnel-to-growth plate ratio of less than 3% in both the femur and the tibia.²⁰ These numbers are important in light of data from a study performed by Guzzanti et al.⁸ in rabbits, which demonstrated injury involving 3% of the cross-sectional area of the femur and 4% of the tibial cross-sectional area as yielding low risk for subsequent deformity. No femoral deformities occurred, and 3 of 21 developed tibial deformities but no epiphysiodesis occurred.⁸

All-epiphyseal techniques require intraoperative fluoroscopy, as the tunnels must be assured to be below the femoral physis. If errant tunnel placement occurs, this could destroy up to 50% of the physis, as the tunnel lies parallel to the physis. An anatomic study of pediatric and fetal femurs demonstrated that the femoral physis was on average about 3 mm from the femoral ACL insertion. The data from this study suggest that transverse tunnels attempting to stay distal to the femoral physis (all-epiphyseal technique) would require at maximum a 5-mm-diameter reamer or less and provide for no margin of error before physeal destruction would occur.²¹

Animal Model Studies

Animal studies can help us better understand the growing physis; however, the typical models use rabbits and dogs whose physes grow faster than in children. Such differences in growth rates can confound some findings and possibly exaggerate any deformity noted. The previously mentioned study by Guzzanti et al.⁸ used a rabbit model and found minimal damage to the physis, yet 3 of 21 developed tibial deformity despite no physeal bridge forming. Another study using a canine model cautioned against overtensioning of the graft. In this beagle model, significant valgus deformity of the distal femur and varus deformity of the proximal tibia occurred despite no physeal bar formation when the ACL graft was overtensioned.²² A study using a canine model demonstrated a protective effect of placing a soft tissue graft across the growing physis. Using fascia lata, the knees with a soft tissue graft had no evidence of bony physeal bridge, whereas the control group where tunnels were drilled and no graft was placed developed a physeal bony bridge.⁶ Similar results were demonstrated in a rabbit study where empty tunnels produced a bone bridge, whereas tunnels filled with a soft tissue graft reduced the development of bony bridges and incidence of angular deformity. The addition of mesenchymal stem cells, however, was protective against the development of bone bridges or angular deformities.²³

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