Most ACL injuries in the pediatric population result from osteochondral avulsion fractures of the tibial eminence.1,4,5
suggested that midsubstance tears in youths are not uncommon and are increasingly likely as the child ages from preadolescence to adolescence. Although osteochondral avulsions from the femoral attachment site of the ACL are uncommon, clinicians must be aware of these injuries in order to recognize them quickly and begin immediate appropriate treatment. Anterior cruciate ligament osteochondral avulsion fracture treatment focuses on bony union rather than a reconstructed ligament, so prompt diagnosis and management are essential because a delay could lead to malunion. Additionally, failing to diagnose the ACL-deficient knee will undoubtedly lead to knee instability and, as noted by Millett et al,19
increase the incidence of associated knee injuries (eg, medial meniscus tears).
A review of the literature reveals 2 cases of cartilaginous9,15
and 5 osteochondral10–14
avulsion fractures from the femoral origin of the ACL. As did our patient, the 7 previously documented patients underwent operative intervention for the avulsions. All 5 osteochondral avulsions resulted from high-energy mechanisms, including snow skiing (both children were 11 years old),11,13
falling from motorized moving vehicles (patients were 12 and 13 years old),12,14
and falling from the monkey bars while getting a leg caught in the rungs (7-year-old child).10
Although our patient's injury resulted from what Rice20
identified as a lower-energy mechanism (ie, prepubertal athletics), it yielded a similar result. Treatment for injuries resulting from the 2 mechanism types is similar, and medical personnel caring for pediatric athletes should be aware that even low-energy mechanisms can result in an ACL-deficient knee.
Early biomechanical studies of the ACL's function revealed the ligament to be taut from full extension to 20° of flexion and again from 70° to 90° of flexion, with a period of relaxation occurring between 40° and 50°. Additionally, increased ACL tension was found throughout the knee arc of motion with tibial internal rotation.21
Boden et al22
noted that most ACL injuries occurred at footstrike with the knee close to full extension. Subgroup analysis showed that noncontact mechanisms consisted primarily of sudden decelerations with a landing motion or a planned change of direction, whereas contact mechanisms typically consisted of a valgus collapse of the knee.22
have shown that the ACL's role in anterior tibial translation resistance during deep knee flexion is aided by other surrounding soft tissues (eg, medial collateral ligament), but the common perception is that it acts in near isolation for such resistance near full extension. Even though our patient did not recall the exact mechanism of injury, this biomechanical understanding may help us reconstruct the probable mechanism. Upon tackling our patient, whose injured foot may have been planted, his opponent probably created an internal tibial rotation or valgus force (or both) while the patient's knee was near full extension, causing enough tension to tear most ACLs. However, because the patient was skeletally immature, his ACL was stronger than its associated physeal insertion site, making him prone to an avulsion fracture.
The role of MRI in ACL avulsion fractures has not been studied well to date.25
Secondary signs of an avulsion fracture, such as a complex hemarthrosis (eg, hemarthrosis or lipohemarthrosis) () can aid in the diagnosis. Stanitski et al17
demonstrated that hemarthroses were associated with ACL tears 63% of the time. Moreover, Prince et al26
showed that complex effusions seen on MRI correlated highly with avulsion fractures (66% of hemarthroses and 100% of lipohemarthroses). Other modalities for identifying ACL avulsion fractures, such as CT, have shown promising results. Griffith et al27
retrospectively evaluated ACL tibial avulsion fractures and showed that only 48% were visible (ie, fracture margins fully delineated) on radiographs, whereas 100% were visible on CT. Additionally, fragment orientation was better distinguished on CT, including an inverted fracture fragment initially thought to be a tibial avulsion fracture. Although an MRI is an appropriate diagnostic test for most ACL injuries, CT proved to be a key preoperative planning tool for our patient, affecting the specific method of fixation.
Postoperative rehabilitation is integral to the overall success of ACL reconstructive surgery. Stanitski28
noted that rehabilitating the entire lower extremity using isometric, isotonic, and isokinetic exercises contributed to rapid improvement, and modifying the devices or equipment may be necessary for children, given their size (eg, too small for certain machines) or understanding (eg, unable to comprehend proper equipment use). The literature is scarce for rehabilitation regimens after ACL osteochondral avulsion fracture. Our postoperative rehabilitation protocol mimicked that for a bone–patellar tendon–bone ACL reconstruction regimen because both procedures require bony union before significant stress can be placed on the attachment site. The one difference resides in the fixation construct; rather than being able to augment the ACL reconstruction in a bone–patellar tendon–bone graft with another device (eg, interference screw), we had to rely on physiologic healing, which has a time-dependent rate-limiting step. Therefore, we adjusted our patient's postoperative course by immobilizing the knee in full extension for 6 weeks, until bridging callus was identified radiographically.
Criteria for full weight bearing without crutches started with satisfactory pain management and freedom from patellofemoral pain but included quadriceps muscle control, quadriceps and hamstrings cocontractions, and the ability to perform a straightleg raise with an extension lag of less than 2°. Return to full activity has usually been restricted until full ROM was achieved, with restoration of quadriceps-hamstrings strength ratios on concentric and eccentric isokinetic testing and performance of sport-specific tasks at full speed.28
Our criteria required subjective freedom from pain, stiffness, and giving way with activities of daily living, rehabilitation activities, and sport-specific agility drills. Objectively, the patient had to demonstrate full active and passive ROM and no quadriceps extension lag. Comparison testing of the contralateral lower extremity using isokinetic dynamometer, KT 1000 (Medmetric Corporation, San Diego, CA), and functional sport-specific testing must be within 90% of that in the healthy extremity.
Although ACL avulsion fractures of the femoral attachment are rare in the pediatric population, particularly during low-energy sports such as football, prompt diagnosis is essential. Identifying a relevant mechanism of injury, examination findings consistent with a knee effusion and a positive Lachman test, and obtaining appropriate diagnostic studies will expedite suitable treatment. When surgical fracture fixation is accomplished, the postoperative rehabilitation regimen provides the ideal environment for reconditioning with the goal of eventual unhindered return to play.