It is quite common to have a feeling that your knee "locks up" or "catches". The classical treatment for those "mechanical" symptoms has been surgery to take out a meniscus (cartilage).
A recent study published in the Ann Intern Med (Feb, 2016) showed that meniscectomy does not improve pain or function as compared with sham surgery.
Physical therapy seems to be just as good as surgery, if not better. Although it is not known what causes such mechanical symptoms, I suspect that inflammation (non-arthritic), may be involved or a structural problem (misalignment). I recommend osteopathic evaluation/treatment as well as the use of low level ("cold" laser), which are much more cost-effective than surgery.