Background: Anterior cruciate ligament (ACL) injury and re-injury rates are high and continue to rise in adolescents. After surgical reconstruction, less than 50% of patients return to their pre-injury level of physical activity. Clearance for return-to-play and rehabilitation progression typically requires assessment of performance during functional tests. Pain may impact this performance. However, the patient's level of pain is often overlooked during these assessments.

Purpose: To investigate the level of pain during functional tests in adolescents with ACL injury.

Fifty-nine adolescents with ACL injury (ACLi; female n=43; 15 ± 1 yrs; 167.6 ± 8.4 cm; 67.8 ± 19.9 kg) and sixty-nine uninjured (CON; female n=38; 14 ± 2 yrs; 165.0 ± 10.8 cm; 54.2 ± 11.5 kg) performed a series of functional tests. These tests included: maximum voluntary isometric contraction (MVIC) and isokinetic knee flexion-extension strength tests, single-limb hop tests, double-limb squats, countermovement jumps (CMJ), lunges, drop-vertical jumps (DVJ), and side-cuts. Pain was reported on a 5-point Likert scale, with 1 indicating no pain and 5 indicating extreme pain for the injured limb of the ACLi group and non-dominant limb for the CON group, after completion of each test. Chi-Square test was used to compare groups for the level of pain in each test. Analysis of the level of pain within and between groups was performed using descriptive statistics.

The distribution of the level of pain was different between groups for all functional tests (p≤0.008), except for ankle plantar flexion and hip abduction MVICs (Table 1). The percentage of participants reporting pain was higher in the ACLi group in all tests compared to the CON group (Figure 1). Participants most often reported pain during the strength tests involving the knee joint, followed by the hop tests and dynamic tasks, respectively. More specifically, the knee extension MVIC was the test most frequently reported as painful (70% of the ACLi group), followed by the isokinetic knee flexion-extension test, with 65% of ACLi group. In addition, among all hop tests, pain was most often reported during the timed 6m hop (53% of ACLi), and, among all dynamic tasks, during the side-cut (40% of ACLi) test (Figure 1). Furthermore, the tests that led to the higher levels of pain (severe or extreme) were the cross-hop (9.8% of ACLi), CMJ (7.1% of ACLi), and the isokinetic knee flexion-extension test (11.5% of ACLi) (Table 1).

Adolescents with and without ACL injury reported different levels of pain for all functional tasks, except for ankle and hip MVICs. The isokinetic knee flexion-extension test resulted in greater rates of severe or extreme pain and was also the test most frequently reported as painful. Functional tests that frequently cause pain or severe level of pain (e.g., timed 6m and cross hops, side-cut, knee flexion/extension MVICs and isokinetic tests) might not be the first test choices to assess function in patients after ACL injury/reconstruction. Reported pain during functional tests should be considered by clinicians and rehabilitation team members when evaluating a patient's readiness to return-to-play.

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Juvenile Osteochondritis dissecans (JOCD) in humans and subchondral cystic lesions (SCL) in horses (also termed radiolucencies) share similarities: they develop in skeletally immature individuals at the same location in the medial femoral condyle (MFC) and their etiology is only partially understood but trauma is suspected to be involved. JOCD is relatively uncommon in people whereas SCLs arise in 6% of young horses leading to lameness. Ischemic chondronecrosis is speculated to have a role in both osteochondrosis and SCL pathogenesis. We hypothesize that MFC radiolucencies develop very early in life following a focal internal trauma to the osteochondral junction. Our aims were to characterize early MFC radioluciencies in foals from 0 to 2 years old.

Distal femurs (n=182) from Thoroughbred horses (n=91, 0–2 years old), presented for post-mortem examination for reasons unrelated to this study, were collected. Radiographs and clinical tomodensitometry were performed to identify lesions defined as a focal delay of ossification. Micro-tomodensitometry (m-CT) and histology was then performed on the MFCs (CT lesions and age-matched subset of controls). Images were constructed in 3D. The thawed condyles, following fixation, were sectioned within the region of interest, determined by CT lesion sites. Hematoxylin eosin phloxin and safran (HEPS) and Martius-Scarlet-Blue (MSB) stains were performed. Histological parameters assessed included presence of chondronecrosis, fibrin, fibroplasia and osteochondral fracture. An additional subset of CT control (lesion-free) MFCs (less 6 months old) were studied to identify early chondronecrosis lesions distant from the osteochondral junction.

One MFC in clinical CT triages controls had a small lesion on m-CT and was placed in the lesion group. All m-CT and histologic lesions (n=23) had a focal delay of ossification located in the same site, a weight bearing area on craniomedial condyle. The youngest specimen with lesions was less than 2 months old. On m-CT 3D image analysis, the lesions seemed to progressively move in a craniolateral to caudomedial direction with advancing age and development. Seventy-four percent (n=17/23) of the lesions had bone-cartilage separation (considered to be osteochondral fractures) confirmed by the identification of fibrin/clot on MSB stains, representing an acute focal bleed. Fibroplasia, indicating chronicity, was also identified (74%, n=17/23). In four cases, the chondrocytes in the adjacent cartilage were healthy and no chondronecrosis was identified in any sections in the lesions. Nineteen cases had chondronecrosis and always on the surface adjacent to the bone, at the osteochondral junction. None of the subset of control specimens, less than 6 months old (n=44), had chondronecrosis within the growth cartilage.

Early subchondral cystic lesions of the medial femoral condyle may arise secondary to focal internal trauma at the osteochondral junction. The presence of fibrin/clot is compatible with a recent focal bleed in the lesion. Medial femorotibial joint internal forces related to geometry could be the cause of repetitive trauma and lesion progression. In the juvenile horse, and potentially humans, the early diagnosis of MFC lesions and rest during the susceptible period may reduce progression and promote healing by prevention of repetitive trauma, but requires further study.