Introduction: We report an osteoarthritic patient with huge subchondral cyst-like lesions in the Anterior part of distal femur. Deep and large bone defects and severe lateral laxity due to Advanced osteoarthritis was successfully treated with semi-constrained type total knee arthroplasty with long stem.

Case Report: A 70yrs old Female was admitted in our institution diagnosed with severe bilateral Osteoarthritis. The x-rays showed bone on bone Tricompartment OA Knee with Varus Malalignment. She was posted for Single Stage Bilateral Total Knee Replacement and as planned the Left Knee Was Operated first. After exposure, Proximal Tibial, Distal Femoral Cuts and measurement of extension gaps the synovium from the anterior Femur was removed and sizing was done. The AP cut was then proceeded with. We spotted a small Osteochondral Cyst in the Anterior Femur which was curretted to remove the cystic material, which is when we realised that the cyst was large and communicating with the medulary canal. The remaining Femoral preparation was done keeping in mind the risk of iatrogenic fracture and extension Stem was used in the femur. The defect was then packed cancellous bone graft.

Conclusion: If suspected a Preoperative MRI should be done to exclude any subchondral cysts osteochondral defects and any surprise during surgery. Usually one should keep extension stems ready for difficult cases. Operating surgeon should know his implants very well, as in many standard implants extension stems can only be used when distal femur cuts are taken accordingly as 50 Valgus. Mini incision should be avoided because it may fail to reveal such surprises and may land into periprosthetic fractures.

Keywords: Sub-chondral Cyst, Total Knee Replacement, Extension Stems, Osteoarthritis.

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