Anterior cruciate ligament (ACL) tears are diagnosed by combining the patient's history and physical examination but clinical tests (e.g., Lachman, anterior drawer, and pivot shift) are less accurate within the first three weeks of injury. The Lever sign is a clinical test that has shown to have comparable diagnostic accuracy regardless of the time since injury, but this test has not been subjected to a randomised clinical trial and diagnostic values may be overestimated.

Imaging modalities (e.g., MRI) are utilised when clinical diagnosis is not clear but are expensive and delay diagnosis. Hand-held arthrometry is an instrument that can be used in the clinical setting to provide an immediate, objective measure of ACL laxity, but this device has not been adequately validated.

The first aim of this study is to determine the accuracy of hand-held arthrometry for diagnosing ACL tears following acute injury. A reliable and valid device could reduce healthcare costs and expedite appropriate treatment, thereby improving the management of patients following knee injury. The second aim of this study is to determine the diagnostic accuracy of the Lever sign test using a more robust study design than previously employed in other studies.

Approximately 40-50% of patients that present with immediate knee swelling (within 2 hours) following injury have an anterior cruciate ligament (ACL) tear. Potential consequences of an ACL tear include further knee injury, post-traumatic osteoarthritis, and reduced quality of life, therefore prompt, accurate diagnosis is important to expedite treatment and mitigate these risks. Within the first three weeks of injury, clinical tests have lower diagnostic accuracy due to pain, swelling and patient guarding making it difficult to assess the integrity of the ACL with confidence.

Magnetic resonance imaging (MRI) is often performed following knee trauma, as immediate swelling is associated with significant knee injury. However, MRI is costly and can result in a significant delay in diagnosis due to the time it takes for the scan to be performed, reported and acted on. Hand-held arthrometers are clinical instruments that provide immediate, objective measurement of knee laxity, which can then be used to inform decision making. Previous research indicates a side-to-side difference in laxity >3mm is diagnostic for an ACL rupture, with <3mm indicating a partial tear, but this device has not been adequately evaluated in acute presentations. In addition, the accuracy of ACL tests has not been adequately evaluated using robust methodology (randomised control trial) so diagnostic values may have been previously overestimated.

The objective of this study is to determine the diagnostic accuracy of hand-held arthrometry in acute presentations (<3 weeks) using MRI as the reference standard and a prospective design. In addition, a blinded, randomised control trial will be conducted using known ACL- injured and non-injured patients (based on MRI results), to provide robust diagnostic accuracy values for clinical ACL tests.

The hypothesis is that hand-held arthrometry will have superior diagnostic accuracy to the Lachman values reported in the systematic review and meta-analysis of Sokal et al (Sn: 0.89, Sp: 0.62, LR+ 1.79, LR- 0.33). A second hypothesis is that the Lever sign diagnostic accuracy values will be inferior to those reported in the the SR and MA of Sokal et al (Sn: 0.83, Sp: 0.91, LR+ 9.66, LR- 0.18). If arthrometry provides a reliable and valid measure of knee laxity, the device could save significant costs and delays to diagnosis associated with MRI.

Inclusion Criteria:

• History of knee injury (<3 weeks since trauma for the validity study) with immediate swelling (<2 hours) but no fracture on X-ray.
• Participant is willing and able to give informed consent for participation in the study.
• Male or Female, aged 18 years or above.
• No contraindications to arthrometer testing or MRI (see exclusion criteria).
• Healthy contralateral knee; no current or previous history of significant knee injury, or a history of previous minor injury that is symptomatic at the time of recruitment. No previous knee surgery.

Exclusion Criteria:

The participant may not enter the study if they are unable to provide written consent to study participation, or there are contraindications to arthrometer testing or MRI, including the following:

• History of chronic disease or disorder that may put the participants at risk because of participation in the study including non-united fractures, severe peripheral vascular disease, aneurysms, recent (<3 months) radiotherapy or chemotherapy, neurological disorders (e.g., Parkinson's disease), skin conditions at point of testing, severe osteoporosis, malignancy, rheumatoid arthritis.
• Patients with implanted surgical clips or other ferromagnetic material including shrapnel, metallic implants (excluding joint replacements >6 weeks since surgery), non-MRI compatible prosthetic heart valves, surgery within 6 weeks, pregnancy, patients with compromised thermoregulatory systems.
• History of chronic musculoskeletal disease or disorder in either leg.

Blinded