Last reviewed: September 29, 2026

Sometimes, but not always. Feeling unsteady is common after a knee replacement. It can come from weak thigh muscles or from ligaments around the new knee that are too loose.

In a Dutch study of more than 900 people, about 1 in 5 still felt their knee was unstable a year after surgery. Most of them had felt unstable before surgery too. People who felt unstable had more pain and more trouble with daily activities.

In the first days, weak thigh muscles are a common reason. Some nerve blocks used for pain relief weaken these muscles for a time, and in two studies the knee buckled more often with one type of block than another.

True instability means the ligaments around the new knee are too loose, either in some positions or all of them. It is one of the most common reasons a knee replacement is redone, often within the first few years. When the knee is loose while bent, people often describe swelling that keeps coming back, feeling unsteady going down stairs, and aching around the knee. That description comes from expert reviews, and there is no agreed test for instability.

How the bone is cut can matter. If too much bone is taken from the back of the thigh bone, or the top of the shin bone is cut sloping too far backward, the knee can end up looser when bent than when straight. Setting the joint higher or lower than where it was before surgery can also change how tight the ligaments are through the bend. In knees redone for instability, correcting these problems restored stability. This comes mostly from redo studies and expert reviews, and X-rays alone do not reliably show which knees are unstable.

Strengthening and bracing are often tried first, but they have not been tested well. When the knee is truly unstable, a redo operation usually helps. People redone for instability improve less than people redone for other reasons, and more often feel their expectations were not met. Changing only the plastic insert fails more often than replacing the metal parts too.

In general:

  • Unsteadiness in the first weeks usually improves as the thigh muscles recover.
  • Swelling that keeps coming back, or a knee that gives way on stairs, is worth reporting.
  • A knee that gives way with new pain, warmth or swelling needs to be seen promptly.

Drafted with AI assistance from published sources, reviewed and edited by Babak Sheikh, MD.

Sources:

  • Leichtenberg CS, et al. Self-Reported Knee Instability Associated With Pain, Activity Limitations, and Poorer Quality of Life Before and 1 Year After Total Knee Arthroplasty in Patients With Knee Osteoarthritis. J Orthop Res. 2018. https://doi.org/10.1002/jor.24023
  • Thacher RR, et al. Decreased Risk of Knee Buckling With Adductor Canal Block Versus Femoral Nerve Block in Total Knee Arthroplasty: A Retrospective Cohort Study. Arthroplast Today. 2017. https://doi.org/10.1016/j.artd.2017.02.008
  • Fujita Y, et al. Significantly Earlier Ambulation and Reduced Risk of Near-Falls With Continuous Infusion Nerve Blocks: A Retrospective Pilot Study of Adductor Canal Block Compared to Femoral Nerve Block in Total Knee Arthroplasty. BMC Musculoskelet Disord. 2022. https://doi.org/10.1186/s12891-022-05735-6
  • Salehi N, et al. Why Are Total Knee Arthroplasties Failing Today: What Has Changed After Two Decades? J Arthroplasty. 2026. https://doi.org/10.1016/j.arth.2026.02.020
  • Stambough JB, et al. Flexion Instability After Total Knee Arthroplasty. J Am Acad Orthop Surg. 2019. https://doi.org/10.5435/JAAOS-D-18-00347
  • Abdel MP, et al. Stepwise Surgical Correction of Instability in Flexion After Total Knee Replacement. Bone Joint J. 2014. https://doi.org/10.1302/0301-620X.96B12.34821
  • Del Gaizo DJ, Della Valle CJ. Instability in Primary Total Knee Arthroplasty. Orthopedics. 2011. https://doi.org/10.3928/01477447-20110714-46
  • Vajapey SP, et al. Risk Factors for Mid-Flexion Instability After Total Knee Arthroplasty: A Systematic Review. J Arthroplasty. 2020. https://doi.org/10.1016/j.arth.2020.05.026
  • Howie CM, et al. Radiographic Findings in Flexion Instability After Total Knee Arthroplasty. J Knee Surg. 2023. https://doi.org/10.1055/s-0041-1735279
  • Stroobant L, et al. Instability After Total Knee Arthroplasty: A Systematic Review and Meta-Analysis of Treatment Options and Outcomes. Knee Surg Sports Traumatol Arthrosc. 2025. https://doi.org/10.1002/ksa.70065
  • Grayson CW, et al. Functional Improvement and Expectations Are Diminished in Total Knee Arthroplasty Patients Revised for Flexion Instability Compared to Aseptic Loosening and Infection. J Arthroplasty. 2016. https://doi.org/10.1016/j.arth.2016.03.001
  • Rajgopal A, et al. Are the Outcomes of Revision Knee Arthroplasty for Flexion Instability the Same as for Other Major Failure Mechanisms? J Arthroplasty. 2017. https://doi.org/10.1016/j.arth.2017.05.010
  • Piuzzi NS, et al. Patient-Reported Outcome Measures (Pain, Function, and Quality of Life) After Aseptic Revision Total Knee Arthroplasty. J Bone Joint Surg Am. 2020. https://doi.org/10.2106/JBJS.19.01155
  • Cheng R, et al. Clinical Outcomes of Isolated Polyethylene Exchange Versus Full Component Revision for the Management of Instability Following Total Knee Arthroplasty. J Arthroplasty. 2024. https://doi.org/10.1016/j.arth.2024.04.023

This is general information about what published research shows. It is not medical advice and it is not about your situation. Reading it does not create a doctor-patient relationship with me. I am a retired orthopedic surgeon and I do not see patients. Talk with your own doctor before changing anything about your care. If something feels suddenly or seriously wrong, get medical care promptly rather than waiting.

This page is provided as is. I make no guarantee that it is accurate, complete or current, and medical evidence changes. Use of it is at your own risk.

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