So far in this series, we have established that there are no significant criteria in terms of radiographic appearance or synovial fluid aspirate findings that will permit a definitive diagnosis of Cranial Cruciate Ligament Rupture. All that we can say form these tests is that there is evidence of increased (non-inflammatory) synovial fluid production in a stifle displaying varying degrees of Degenerative Joint Disease.
As the key function of the Cranial Cruciate ligament is to limit cranial tibial displacement and internal tibial rotation, the presence of a palpable increase in either of these two components would (in most cases) be a definitive diagnostic finding. There are two main tests of femorotibial stability:
Anterior Drawer Test
In this test we place a thumb behind the fibular head with index finger on the tibial tuberosity with one hand and with the the other hand place a thumb behind the lateral fabella and index finger on the patella. We then push the tibial hand forward without moving the femoral hand. If the tibia slides forwards, this is indicative (in most cases) of cranial cruciate ligament degeneration. This test should be performed in both flexion and extension for the following reason:
The Cranial Cruciate comprises two main bands: The Craniomedial Band and the Caudolateral Band:
The Craniomedial band is taut in both flexion and extension whilst the Caudolateral band is only taut in extension:
If the Anterior Drawer Test is performed only in extension, then partial ruptures (which are most common) may be missed. If the stifle is stable in extension but unstable in flexion, this indicates that partial rupture involving the Craniomedial Band is present as in flexion the Craniomedial Band should provide stability whilst the caudolateral band is loose. For this reason, this test should be performed in flexion and extension.
The test is best performed in anaesthetised patients as the pressure applied by the digits can cause pain and muscle tension can make it difficult to elicit a positive response even in a very unstable stifle. Care in interpreting the findings should be exercised in very young patients and in patients with effusions for other reasons. These patients may show a degree of Drawer, but there will be an abrupt stop to the Drawer motion in these patients which feels different from that felt in the Cruciate deficient stifle.
Tibial Compression Test
This test is often easier to perform in conscious patients but needs some practice to become confident that a negative response is a true finding rather than being due to error in technique. In patients that will bear weight on the affected limb it can often be tested by simply lifting the contralateral limb with a finger placed on the Tibial Tuberosity when the affected limb is loaded. In many patients you will easily appreciate the cranial tibial thrust as the limb is loaded. If the patient is non weight bearing, the procedure is to fix the stifle with one hand whilst upward pressure is applied to the paw of the same limb with the hock on a neutral standing position. This will elicit anterior displacement of the Tibia in most Cruciate Deficient stifles.
The problem with both of these tests is that they may give false negative results. As mentioned above, many patients will present in the partial rupture phase and it may be partial in the craniomedial band with intact caudolateral band. In these patients both of the above tests may prove negative. In Boxers in particular, there tends to be a marked fibrotic response from the joint capsule in the Cruciate Deficient stifle and this fibrosis may result in a degree of palpable stability that mimics the normal stifle. These are the “problem cases” and the clinician may fall back on history, breed, age and lack of contradictory findings to establish a strongly tentative diagnosis in these cases. If further certainty is needed, then arthroscopy or MRI may be used to visualise the cranial cruciate ligament pathology before proceeding to manage the patient with the appropriate surgical technique. In most cases however this is unnecessary as long as all of the tests indicated in this series have been followed and their results interpreted correctly.