BACKGROUND Stiffness after total knee arthroplasty (TKA) remains a frustrating complication for both patients and clinicians, affecting approximately 1.3 to 5.8% of patients. We evaluated the outcomes of manipulation under anesthesia (MUA) with or without arthroscopic lysis of adhesions (aLOA) following primary TKA. METHODS A single-institution retrospective study on patients who underwent an MUA with or without aLOA after 17,000 primary TKAs was performed, identifying 726 MUAs or aLOA + MUAs by 34 surgeons. Inclusion criteria were any patient undergoing MUA or aLOA after primary TKA. Demographics, perioperative variables, and postoperative outcomes were collected. Failure of MUA or aLOA was defined as repeat MUA or aLOA, revision TKA for arthrofibrosis, or failure to gain ≥ 50% of flexion achieved intraoperatively. RESULTS There were 255 MUAs (40%), and 46 (52.3%) aLOA + MUAs were unsuccessful based on our criteria (P = 0.028). The MUAs were performed closer to the index surgery (70.5 versus 207.1 days; P < 0.001), carried a lower all-cause revision rate (6.9 versus 15.9%; P = 0.003), and gained greater knee flexion at the final follow-up (24.6 versus 11.9°). The MUA ROM improved throughout follow-up, while aLOA + MUA plateaued after the second visit. When successful, MUAs gained 33° knee flexion, and aLOA + MUAs gained 27.6°. In unsuccessful cases, MUAs gained 6.1° knee flexion, and aLOA + MUAs lost 0.2°. Unsuccessful MUAs were less healthy, more likely to have had cancer, and had a cruciate-retaining implant. CONCLUSIONS Both MUA and aLOA + MUA are reliable interventions for treating stiffness after primary TKA with expected gains of up to 27 to 33° in successful cases. The MUAs provided better and more consistent improvement in knee motion with less risk. Unsuccessful MUAs or aLOAs typically manifest by the first postoperative visit and fail to recover. Patients undergoing MUA can be expected to have lower absolute knee flexion pre- and postoperatively but achieve greater gains in knee flexion than aLOA + MUA.