Lipomatosis of nerve and osseous nerve territory overgrowth patterns affecting knee: illustrative cases.

backgroundLipomatosis of the nerve (LN) is a rare peripheral nerve pathology characterized by abundant proliferation of fat within the epineurium giving it pathognomonic features on MRI. Any nerve may be affected, but major nerves such as the median and sciatic nerves are most reported. One of the clinical hallmarks of LN is associated bony and soft tissue overgrowth occurring distal to the nerve lesion. While macrodactyly and limb length overgrowth are well known in LN, this study aims to look at nerve territory bony overgrowth affecting the knee. OBSERVATIONS: An initial search revealed 59 cases of LN in the Mayo Clinic system. Of 19 patients with LN of the lower limbs, 12 cases involved nerves in the pelvis, buttock, or thigh. Three cases fulfilled the criteria (LN of lower extremity with available MR images and knee radiographs) that affected the femoral, sciatic, and combined femoral/sciatic nerves. Knee radiographs revealed bony overgrowth in the respective nerve territories in each case: anterior, posterior, and both anterior and posterior portions of the knee. LESSONS: These illustrative cases of LN with overgrowth of the knee demonstrate that bone overgrowth follows the territory of the associated nerve in LN, which supports the previously published evidence on LN-associated nerve territory overgrowth. https://thejns.org/doi/10.3171/CASE25132.