The 45 degree PA flexion weightbearing view is the best for demonstrating subtle joint-space loss, especially in the lateral compartment. This is because the earliest loss of cartilage occurs in the 30 to 60 degree flexion zone which is easily overlooked on x-rays in full extension. Weight-bearing views are always preferrable when evaluating for arthritis. Cole and Harner describe the evaluation of young active patients with early knee arthritis. Dervin et al. compared AP and 45 degree PA views with results from arthroscopy and found the latter to be superior for detecting lateral compartment wear and concluded that the 45 degree PA should be considered as "the screening radiograph of choice in evaluating osteoarthritis of the knee".
Cole BJ, Harner CD. Degenerative arthritis of the knee in active patients: evaluation and management.J Am Acad Orthop Surg. 1999 Nov-Dec;7(6):389-402. Review.
PMID:11505927 (Link to Abstract)
Dervin GF, Feibel RJ, Rody K, Grabowski J. 3-Foot standing AP versus 45 degrees PA radiograph for osteoarthritis of the knee. Clin J Sport Med. 2001 Jan;11(1):10-6.
PMID:11176140 (Link to Abstract)