Palpation alone is accurate in diagnosing canine ACL tears only in dogs with acute complete tears. Nearly half of all dogs with ACL tears cannot be diagnosed by palpation alone.
Radiographs (X-Rays) are helpful in ACL tear diagnosis. Because of the canine sloping tibial plateau, the femur (top bone) slides down and back on the sloping tibial plateau (bottom bone) during weight bearing. In some instances the lateral radiograph will actually catch the joint in this slipped position. This sliding motion (demonstrated in the image to the left) is one underlying factor leading to canine ACL tears. It is why dogs with untreated ACL tears usually do poorly compared to humans. It is also the idea behind the tibial plateau leveling osteotomy (TPLO).
ACL tears are best diagnosed by an arthroscopic examination at the start of surgery. Arthroscopy is extremely accurate and non-invasive. In this day and age, virtually no open surgical procedure should be performed prior to obtaining an accurate, non-invasive arthroscopic diagnosis.