Unicompartmental knee arthroplasty (UKA)

UKA targets isolated single-compartment osteoarthritis in patients with an intact anterior cruciate ligament, functional collateral ligaments, and a correctable deformity. By preserving native bone stock, cartilage, and soft-tissue structures in the unaffected compartments, UKA typically delivers faster rehabilitation, lower perioperative blood loss, and a more natural post-operative range of motion compared with TKA. Patient selection is the primary determinant of success; implanting UKA in a candidate with multi-compartmental disease or significant malalignment is a leading cause of early revision.

From a coding and reimbursement standpoint, UKA occupies its own CPT code (27446) and its own DRG weight, which differs from the TKA DRG. This separation matters because the implant cost, operative time, and expected length of stay are all lower for UKA—payors use these benchmarks to set reimbursement rates and to trigger medical-necessity reviews. Misidentifying a UKA as a TKA (or vice versa) in the operative note or on the claim creates a mismatch that can produce a denial, a post-payment audit, or an overpayment demand.

When a UKA fails and must be converted to a TKA, the coding path is not straightforward. No CPT code exists solely for UKA-to-TKA conversion. Current authoritative guidance from AAOS and CPT Assistant supports two approaches depending on complexity: CPT 27447 with modifier 22 when primary implants suffice, or CPT 27487 when the reconstruction rises to the complexity of a full revision (removal of existing components plus femoral and entire tibial component reconstruction). The treating surgeon's operative documentation must clearly support whichever path is chosen.

Coding a UKA as a TKA (CPT 27447 instead of 27446) constitutes upcoding—a billing error that exposes the practice to payor audits, claim recoupment, and potential False Claims Act liability. Conversely, coding it as TKA when only a single compartment was addressed may also trigger a medical-necessity denial because the diagnosis codes submitted (e.g., localized medial compartment OA) won't support whole-joint replacement. On the revision side, choosing between CPT 27447-22 and CPT 27487 carries a meaningful reimbursement difference; picking the wrong code without robust operative documentation to back it up is a primary target for orthopedic-specific coding audits.