ACL injuries in children and adolescents often present with symptoms that are similar to those in adults. When interviewing pediatric patients, it is important to keep the patients actively involved in the interview process, even in cases where parents are able to provide much of the salient clinical information. Most partial injuries of the anterior cruciate ligament result from noncontact twisting injuries during sporting activities, rather than slightly higher energy collisions. Again, position of the knee will likely determine which portion of the ACL is ruptured, with AM bundle susceptible in flexion and PL in extension. The mechanism of injury involves the femur being externally rotated on a fixed lower extremity or the tibia being internally rotated relative to the femur as a valgus moment is applied to the knee joint [16, 24, 25]. Patients with partial ACL ruptures frequently recall an audible or appreciable sensation of a “pop” or tearing at the time of injury and generally are incapable of continuing in their sporting activity. After removal from their sporting activity, they frequently experience generalized knee pain that is difficult to localize, swelling, and difficulty with weight bearing activities and may experience a sensation of knee instability, which may be attributable to their ligamentous injury or the resultant joint effusion [9, 16, 26, 27]. Frequently, patients may report an inability to achieve terminal extension, which may be the result of avoiding pressure on the associated bone bruises or the mechanical phenomenon of a ruptured fragment extruding into the lateral joint space or adhering to the infrapatellar fat pad [28, 29].

Physical examination is particularly crucial in cases of suspected partial ACL rupture, as the accurate clinical diagnosis is largely based on the degree of laxity detected on examination. Examination of children, specifically those presenting acutely after injury, can be difficult. Every examination should start with the contralateral extremity to put the patient at ease, permit an explanation and demonstration of relevant physical examination maneuvers, and also allow appreciation of native tissue laxity. When evaluating the injured extremity, always check for gross signs of trauma, including a hemarthrosis or effusion. A hemarthrosis commonly develops but is not required for diagnosis [30, 31].

Measurement of the anterior translation and rotatory stability of the injured knee is paramount. The anterior drawer, Lachman, and pivot shift are the most common methods used to assess the anterior translation and rotatory stability of the knee. Typically, these measurements quantify the degree of laxity present in the injured extremity and compare it both to normative thresholds as well as to the contralateral, uninjured extremity as an internal control [9, 32, 33].

When gauging anterior translation laxity, the Lachman test has superior sensitivity when compared to the anterior drawer test, and previous biomechanics studies have noted that the ACL undergoes greater strain at 30° of flexion compared to 90° [10, 11, 34]. Sectioning of the ACL also results in more anterior translation at 30° of flexion than at 90°. Of note, neutral rotation must be maintained to avoid interference by secondary stabilizers [34]. While the anterior drawer test is commonly described and performed as a means to assess ACL function, it can be an unreliable measure of anterior knee laxity [35, 36]. The anterior drawer test is complicated clinically by difficulty flexing the knee to 90° due to pain in acute cases, spastic contraction of the hamstring muscles dampening exam results, and greater support against anterior translation by secondary stabilizers at the 90° flexion position (including the osseous contour of the joint, the posterior meniscal horns, and the medial collateral ligament). Indeed, some authors feel that when an anterior drawer test is positive, secondary injuries should be considered [9].

While measurement of anterior translation is essential when considering ACL injuries, cadaveric and clinical data has called into question the reliability of the Lachman and anterior drawer exams in evaluating partial ACL injuries. Numerous studies have showed difficulty in distinguishing partial ACL tears from intact ACLs on examination alone [11, 18, 37]. Hole et al. showed that physical exam and KT-1000 testing was incapable of distinguishing between an intact ACL and a partial rupture involving an entire functional bundle (PL). They also noted that it was unlikely to distinguish between a fully ruptured ACL and one with a 75% partial rupture (entire PL and 50% AM). In a similar study, Lintner et al. found that sectioning of the AM bundle did not examine differently compared to an intact ACL, again using physical examination and KT-1000 testing [18].

The pivot shift test , therefore, is considered superior to both the anterior drawer and Lachman tests for defining anterior cruciate ligament insufficiency, particularly functional insufficiency. The pivot shift phenomenon results in a subluxation/reduction mechanism present only in ACL-deficient knees (either complete ruptures or “nonfunctional” partial ruptures). In ACL-deficient knees, the femur sags posteriorly to the tibia in extension due to gravitational pull. This position brings the iliotibial band anterior to the center of rotation of the knee in extension, but as the knee is flexed and the gravitational pull is negated, the femur spontaneously reduces atop the tibia. A palpable clunk is noted as the iliotibial band passes behind the center of rotation of the knee [38]. The test result is graded as 1+ (glide), 2+ (jump), or 3+ (transient lock). A positive result is indicative of anterior cruciate ligament deficiency, either by complete rupture or nonfunctional partial rupture. This test is especially useful for identifying anterior laxity that will become clinically disabling [8, 35, 38]. When specifically considering children, Kocher et al. highlighted subtle laxity on pivot shift testing (normal versus 1+ glide) as a strong predictor for the need for subsequent ACL reconstruction in arthroscopically diagnosed partial injuries in the pediatric age group. The major limitation of the pivot shift test is that it tends to be the most painful exam maneuver in the ACL evaluation. Especially in children, significant guarding should be expected and clinic testing may not provide an accurate representation of laxity. The exam can be repeated a few weeks after injury once the acute pain has abated; however, numerous studies recommend an evaluation under anesthesia to obtain a true sense of pivot shift stability [8, 38].

KT-1000 arthrometer testing diagnostic of anterior cruciate ligament deficiency is defined as a maximum side-to-side difference of >3 mm, a maximum manual translation of >10 mm, or a compliance index of >2 mm with application of a 20-lb. force at 30° of knee flexion [39]. The primary author of that arthrometric study has later suggested that low-grade side-to-side differences (i.e., <3 mm) in KT-1000 measurements are suggestive of a partial ACL rupture and that side-to-side discrepancies >3 mm are very rarely found with partial injuries [9]. Cadaveric data corroborates this idea in showing that the average anterior laxity with partial ACL rupture is 1.3 mm [18]. Regardless, when the difference between partial and complete rupture measurements lies between 1.3 mm and 3 mm, the diagnostic strength of the tool is moderate at best.

Few clinical studies have included arthrometric data when evaluating partial ACL ruptures. Noyes et al. showed a trend toward progressive ACL deficiency in patients presenting with an initial side-to-side difference in laxity of >5 mm. Bak et al. reported that 71% of patients had a mean side-to- side difference of <2 mm with all remaining patients found to have a difference < 4 mm [40]. Fruensgaard and Johannsen reported an average difference of 2.8 mm in patients with clinically unstable knees and 2 mm in patients with clinically stable knees [41]. Other studies have shown conflicting results of low-level side-to-side differences (<3 mm) at injury presentation in predicting long-term stability of the knee and need for eventual reconstruction [37, 42, 43].

Newer arthrometer technology has shown improved capabilities of distinguishing between complete and partial ACL ruptures in isolated studies. Robert et al. showed a sensitivity of 80% and specificity of 87% with the use of the GeNouRoB arthrometer in accurately identifying partial ACL ruptures [44]. No studies are available regarding arthrometric data in children with partial ACL ruptures. Overall, arthrometric testing of patients is a reasonable tool to evaluate anterior translation but lacks the sensitivity and predictive power to be diagnostic. As a result, it has not found widespread application in most clinical settings.

MRI is the standard diagnostic tool to detect an injury to the ACL. MRI allows visualization of the ACL itself along with the other articular structures about the knee that are at risk in ACL-deficient athletes, including the menisci, articular cartilage, the medial collateral ligament (MCL), and structures of the posterolateral corner (PLC). It also allows visualization of the lateral-sided bone bruise patterns typical in cases of ACL rupture. While MRI has become the diagnostic imaging standard for ACL injuries, standard MRI performs poorly when attempting to discern complete versus partial ligament ruptures. Multiple studies have documented low sensitivity and specificity of conventional MRI in diagnosing partial thickness ACL ruptures [45–48]. In the lone study specific to pediatric patients, MR sensitivity was 71% for diagnosing partial ruptures [8].

The low diagnostic utility of MRI arises specifically from difficulty distinguishing between complete and partial ruptures. Both diagnoses feature high signal changes within the ligament, mass effect, abnormal ligamentous contour, and fiber discontinuity. However, to diagnose a partial rupture, it is imperative to show contiguous fibers [49]. Due to the oblique course of the native ACL, the typical axial, sagittal, and coronal MRI sequences do not adequately capture the ligament in full, making determination of fiber integrity difficult [50, 51]. Several studies have been performed with the inclusion of oblique imaging sequences in an effort to better capture the anatomic course of the ACL and have shown modest improvements over conventional MR imaging [45, 50, 52, 53]. Isolated studies evaluating the effect of magnet strength (1.5-T vs. 3-T) and novel imaging sequences have also been reported with varying degrees of success [54–57]. Clearly, more research is needed to improve our imaging capabilities for accurately identifying these injuries.

Diagnosis of partial ACL ruptures is quite difficult based on patient history, examination, and ancillary diagnostics, as discussed previously. As such, physicians may elect to perform a diagnostic arthroscopy in cases of suspected partial ligament ruptures to provide a definitive diagnosis [8, 18, 27, 29–31]. This option has multiple benefits, as it also permits an examination under anesthesia to best evaluate the functional competency of the ACL and permits a direct visualization of the ACL to discern the degree of ligament injury. Noyes et al. showed that diagnostic arthroscopy was the most accurate means of assessing the degree of ligament damage and showed that the degree of tear predicted the future development of insufficiency symptoms [27]. Kocher et al. showed that diagnostic arthroscopy was accurate in evaluating the degree of ligament injury and showed that ruptures with >50% of ligament involvement had a significantly higher rate of requiring delayed reconstruction. This study also demonstrated that anatomic location of tear was also important in a pediatric population, with posterolateral tears requiring eventual reconstruction at a higher rate [8].

While these studies have shown arthroscopic grading to be predictive of outcomes, interestingly, other studies have shown no correlation [41, 43, 58]. This juxtaposition of evidence highlights the difficulty in predicting overall function of the ACL after injury. As mentioned previously, normal-appearing ACL tissue may mask a substantial injury through either microscopic plastic deformation that prevents normal function or through providing misleading appearance of normality on arthroscopic evaluation. This scenario can arise when the ligament’s synovial sheath remains intact despite a sizable ligamentous rupture, normal appearance of the tibial insertion anatomy despite injury at the femoral wall origin, or scarring of injured ligament to the PCL, intercondylar notch, or other surrounding synovial tissues, giving the false appearance of tissue integrity. In each of these cases, the degree of injury is underestimated and can lead to a missed diagnosis of substantial injury [59]. This evidence suggests that arthroscopy may be used as a valuable tool in the diagnosis of partial ACL ruptures but should not be used in isolation to determine the eventual treatment pathway for patients. Instead, arthroscopic results should be considered in the context of the physical examination and imaging data to help refine the treatment of choice.

As described in the preceding sections, there is no perfect means by which to accurately diagnose a partial ACL tear, let alone determine its future functional performance. Noyes used arthroscopic findings, with partial tears classified as <25%, 50%, or 75% ligament ruptures, to determine the need for reconstruction [27]. DeFranco and Bach used a combination of findings to determine severity of injury, diagnosing [9] “functional” partial ACL ruptures based on the following four criteria: (1) asymmetry on Lachman testing, (2) a negative pivot shift test on exam under anesthesia, (3) a low-grade KT-1000 arthrometer measurement (<3 mm), and (4) arthroscopic evidence of a partial anterior cruciate ligament injury. If the four qualities are met on a patient evaluation, the patient is diagnosed with a partial ACL rupture that has maintained the functional integrity of the ACL. Their system relied primarily on the pivot shift examination under anesthesia as the benchmark for determining ligamentous stability, as an asymmetric positive pivot shift of any magnitude indicated functional incompetence of the ACL, for which ACL reconstruction was advocated.

The pediatric literature by Kocher et al. showed outcomes results that reflected elements of the approaches of both Noyes and DeFranco/Bach. Their study noted that arthroscopic size of rupture was predictive of eventual need for reconstruction, with tears >50% failing nearly three times more frequently than those involving <50%, which parallels Noyes’ observations. They noted a similar finding with anatomic location of tear at arthroscopy, with PL tears undergoing reconstruction three times more frequently than AM tears. The study also found trends toward instability predicting future need for reconstruction depending on examination findings. One hundred percent of patients with a subtly abnormal pivot shift examination (5/5 patients) underwent eventual reconstruction, while only one in four patients with a normal pivot shift required eventual reconstruction, similar to DeFranco and Bach’s guidelines. On Lachman testing, patients with <3 mm difference underwent subsequent reconstruction in 9% of cases with patients with 3–5 mm difference required reconstruction in 38% of cases. Lastly, the study noted that age was a significant predictor, with older adolescents (>14 years old) which were found to be more likely to progress to ACL insufficiency symptoms than their younger counterparts [8].

Overall, we recommend a multimodal approach to diagnosing partial ACL ruptures and prescribing a specific treatment pathway. All patients who are felt to have a partial thickness ACL rupture due to injury mechanism and subsequent symptoms and examination findings, such as subtle asymmetric laxity, should receive an MR evaluation to evaluate the status of the ACL and other soft tissue structures of the knee. Based on this data, patients felt to have a clinically stable knee should enter a non-operative treatment pathway, which is described in later sections. Patients with borderline exam and MR findings concerning for potential ACL insufficiency should undergo an operative procedure involving an examination under anesthesia and diagnostic arthroscopy. Signs of ACL insufficiency on examination under anesthesia, specifically a positive pivot shift of any grade, should undergo subsequent reconstruction. If the knee proves stable on examination under anesthesia, arthroscopic findings with regard to size and location of tear should be considered in the context of the young athlete’s functional demands to determine non-operative versus operative treatment. One of the challenges of this overall approach is the discussion with the family and patient, who may wake up with two extremely different treatment pathways pursued and postoperative courses. However, a thoughtful preoperative explanation of the goals of achieving the optimal balance of each patient’s functional goals, while being mindful of both ligament preservation and joint preservation principles, should help families understand the complex nature of management of this specific subset of injuries.

Non-operative management should be reserved for patients with partial ACL tears that maintain the basic native function of the ligament. Kocher et al. described the non-operative pathway used in his pediatric cohort as follows: (1) weight bearing restriction of touchdown only for 6–8 weeks then advanced as tolerated, (2) a hinged knee brace was used to prevent hyperextension (the study protocol avoided passive terminal extension for 6 weeks and active terminal extension for 12 weeks), (3) a physical therapy program was started early in the recovery process and focused on hamstring strengthening for dynamic support, and (4) patients typically returned to play at 3 months from the time of injury and were expected to use a brace [8].

The majority of adult literature also endorses a brief period of limited weight and motion restriction accompanied by a progressive rehabilitation program [27, 30, 32, 33, 37, 40, 43]. Other authors advocate an accelerated course including early progressive weight bearing and initiation of a physical therapy protocol without motion restriction with the goal of preventing arthrofibrosis [9]. Overall, a rehabilitation program should include stepwise progression through range-of-motion exercises, lower extremity and core muscle strengthening, cardiovascular endurance training, perturbation training, and sport-specific skill training [60, 61]. Prior to clearance for return to sport, strength and functional performance tests should be performed to ensure the athlete’s rehabilitation is optimized [62]. It is essential to maintain close observation on these patients throughout the rehabilitation and return-to-play period to ensure that functional knee stability is maintained.