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After a brief description of the functional anatomy of the stifle joint, the literary section enters into the types and pathophysiology, clinical and radiographic diagnosis and the therapy of the congenital patellar luxation in the dog.From 1997 to 1999 at the clinic and polyclinic for small domestic animals at the Free University Berlin 74 stifle joints of 37 dogs of different breed, age and sex were examined. The clinical degrees of patellar luxation of the pelvic limbs were determined according to the classification designed by PUTNAM (1968). Lateral radiographs were made with a microfocus X-ray tube using the direct magnification technique for an indirect evaluation of the trochlear depth and for measuring the patellar thickness. During arthrotomy an impression of the trochlear groove was performed on 29 stifle joints. This impression was used to build a plastercast of the trochlear surface for a direct measurement of the trochlear depth. The comparison of the results of the trochlear depth on the radiographs and the plaster-casi showed an approximative linear regression.The measured parameters on the microfocus radiographs were used to figure out the quotient of the trochlear depth and the patellar thickness. The Q-angle according to KAISER (1999) was measured on ventrodorsal pelvic radiographs. The results of the measured parameters were compared with the clinical degrees of patellar luxation. While the results of the Q-angle corresponded with the values reported by KAISER (1999), here is no agreement with the literature in relation to the values of the quotient of trochlear depth and patellar thickness. In this work, the ascertained quotients were much lower than reported in the existing literature. A definite relation between trochlear depth and the clinical degrees of patellar luxation could not be established. However, it was possible to explain patellar instability in dogs with congenital unilateral luxation by the indirect evaluation of the trochlear depth in combination with the Qangle. In consideration of the clinical examination, both parameters make a preoperative planning of the surgical corrections at the skeleton possible, which might lead to a better prognosis.