Specific Risks of Total Knee Replacement

Part of The Joint Replacement Guide — Phase 3: Understanding the Risks · Article 16 of 44 · Knee

Key Points

  • Risks particularly associated with knee replacement include stiffness, persistent pain, instability, kneecap problems, component-position problems and fracture around the implant.
  • Knee replacement reliably improves pain and function for most patients, but long-term discomfort is reported more often than after hip replacement.[1]
  • Approximately four out of five patients report being satisfied after knee replacement, although estimates vary according to how satisfaction is measured.[2]
  • Early stiffness may improve with time and appropriate rehabilitation. Some patients require manipulation under anaesthesia or further surgery.
  • Knee components can be positioned using different alignment philosophies. An X-ray that does not look perfectly “straight” is not necessarily abnormal.
  • Revision surgery may be required if a significant mechanical or biological problem develops, but it is not the usual outcome.

Stiffness

Some swelling, tightness and difficulty bending or straightening the knee are expected during early recovery. Improvement usually continues over several months, and the final movement cannot be judged in the first few weeks.

More substantial or persistent stiffness can be associated with:

  • limited movement before surgery;
  • pain and swelling after surgery;
  • excessive scar formation, known as arthrofibrosis;
  • infection;
  • component position or size;
  • instability or ligament imbalance;
  • another medical or neurological condition; and
  • interruption or difficulty with rehabilitation.

Stiffness should not automatically be blamed on a patient for “not trying hard enough”. Pain, swelling, wound problems, access to physiotherapy and mechanical causes can all limit exercise. Persistent stiffness requires assessment rather than simply increasingly forceful exercise.

Treating Stiffness

Initial treatment may include:

  • reviewing pain relief;
  • managing swelling;
  • an appropriate exercise programme;
  • physiotherapy where indicated; and
  • investigating infection or a mechanical problem if symptoms are atypical or progress is poor.

Physiotherapy can help many patients, but it cannot correct every cause of stiffness. Exercises should be purposeful and regular without being so forceful that they repeatedly cause severe pain or marked swelling.

Manipulation under anaesthesia

If movement remains substantially restricted despite appropriate initial treatment, the surgeon may consider a manipulation under anaesthesia, or MUA. While you are anaesthetised, the surgeon carefully moves the knee to improve its range.

MUA is often considered during the first three months, but there is no single deadline appropriate for everyone. Earlier MUA is generally associated with a larger improvement in movement and fewer subsequent interventions than delayed MUA, although patient selection affects these comparisons.[3]

Before recommending MUA, the team should consider whether infection, component malposition, instability or another mechanical problem is contributing. MUA may improve movement, but it does not guarantee a normal range and carries uncommon risks, including fracture, bleeding, wound problems or injury to the extensor mechanism.

Persistent stiffness may occasionally require keyhole release of scar tissue, open surgery or revision knee replacement. Outcomes from revision undertaken for stiffness can be unpredictable, so the likely benefits and risks require careful discussion.[4]

Persistent Pain and Discomfort

Knee replacement is effective for most people, but it cannot guarantee a completely pain-free or natural-feeling knee.

Research has often estimated that approximately 20% of patients experience some degree of long-term pain after total knee replacement.[1] This figure includes different types and severities of pain, and should not be interpreted as meaning that one in five replacements has mechanically failed.

Satisfaction and persistent pain are also not identical. A person can experience some residual discomfort while still being pleased with the overall improvement. Conversely, someone may have little pain but remain dissatisfied because movement or function does not meet their expectations.

A systematic review found that more than 80% satisfaction was reported in most published studies, but methods of measuring satisfaction varied considerably.[2]

Possible causes

Persistent pain can arise from:

  • infection;
  • loosening or wear;
  • instability;
  • stiffness;
  • component position, rotation or size;
  • kneecap or tendon problems;
  • nerve-related pain;
  • pain referred from the hip or spine;
  • sensitivity of the nervous system after longstanding pain; or
  • another condition unrelated to the implant.

Pain should not be dismissed simply because an ordinary X-ray appears satisfactory. Assessment may require examination, blood tests, comparison with previous images or, selectively, further imaging.

Equally, further surgery should not be undertaken solely because pain persists unless there is a reasonable diagnosis and a meaningful prospect that surgery will help. Revision for unexplained pain has less predictable results.

Instability

A knee replacement depends on the components, ligaments and surrounding muscles working together. Instability means that the knee has excessive or poorly controlled movement. It may feel as though it shifts, buckles or gives way.

Possible causes include:

  • unequal ligament tension;
  • damage or gradual stretching of a ligament;
  • component position or rotation;
  • an imbalance between the bending and straightening spaces within the knee;
  • polyethylene wear;
  • muscle weakness; and
  • the design or level of constraint of the replacement.

Not every episode of “giving way” represents implant instability. Pain, weakness, poor balance or a neurological condition can create a similar sensation.

Mild symptoms may sometimes be managed with strengthening, activity modification or a brace. Physiotherapy cannot correct substantial mechanical instability. Persistent instability that causes pain, recurrent swelling or loss of confidence may require revision surgery.[5]

Alignment, Component Position and Rotation

Knee-replacement components have to be positioned in three dimensions. The surgeon considers:

  • the overall alignment of the leg;
  • the joint line;
  • the position and size of the femoral and tibial components;
  • rotation of each component;
  • ligament and soft-tissue tension;
  • stability through the range of movement; and
  • how the kneecap tracks.

Different alignment philosophies

Traditional mechanical alignment generally aims to position the leg around a neutral mechanical axis. Newer approaches—including kinematic, restricted kinematic and functional alignment—may place the components with greater reference to the patient’s pre-arthritic anatomy and individual soft-tissue balance.[6]

Many people naturally have some degree of bow-legged or knock-kneed alignment. A personalised strategy may intentionally preserve part of this individual anatomy within defined safety limits rather than making every knee radiographically identical.

Consequently, a postoperative X-ray may look unfamiliar or not perfectly straight to a non-specialist without representing an error. It should be interpreted in relation to:

  • the surgeon’s intended alignment strategy;
  • preoperative anatomy;
  • component position;
  • clinical stability;
  • symptoms; and
  • change over time.

Different accepted alignment strategies have produced broadly comparable complication rates in current studies, although their relative long-term advantages remain under investigation.[6]

Malalignment and malrotation

An individualised or non-neutral alignment must not be confused with unintended malalignment. Clinically important malalignment or malrotation can contribute to:

  • pain;
  • stiffness;
  • instability;
  • uneven loading;
  • kneecap maltracking; and
  • premature wear or loosening.

Rotation is difficult to assess reliably on a routine front or side X-ray. If symptoms and examination raise a genuine concern, specialised imaging such as a CT scan may sometimes be considered. Imaging findings must still be interpreted carefully, because measurement methods and normal ranges vary.

Published research associates particular patterns of component malrotation with patellar maltracking, pain, instability and stiffness, but an imaging measurement alone does not automatically establish the cause of a patient’s symptoms.[7]

Kneecap and Extensor-Mechanism Problems

The kneecap moves through a groove at the front of the femoral component. Problems can occur if its movement is affected by component position, soft-tissue balance, scar formation or the condition of the kneecap itself.

Possible complications include:

  • pain at the front of the knee;
  • clicking or catching;
  • tilt, maltracking or instability of the kneecap;
  • wear or loosening of a resurfaced patellar component;
  • fracture of the kneecap; and
  • rupture of the quadriceps or patellar tendon.

Some mechanical sensations from a knee replacement are painless and do not require treatment. New pain, swelling, instability or difficulty actively straightening the knee requires assessment.

Physiotherapy may help when weakness or movement control contributes, but it cannot correct every mechanical problem. Significant extensor-mechanism injury is rare but serious and may require surgery.

A kneecap fracture can occur whether or not the back of the patella has been resurfaced, although resurfacing introduces additional implant-related considerations.

Fracture Around the Knee Replacement

A fracture can occur during implantation or later around the femoral, tibial or patellar components. A later fracture around an implant is called a periprosthetic fracture.

Risk may be higher with:

  • osteoporosis or poor bone quality;
  • increasing age or frailty;
  • previous surgery;
  • certain implant or fixation factors;
  • a loose component; and
  • a fall or other injury.

Treatment depends on the fracture, implant stability, bone quality and general health. A small stable fracture may sometimes be managed without further surgery. Other fractures require plates, screws, a rod inside the bone or revision to a different implant.[8]

After a fall, seek urgent assessment for marked new pain, deformity or inability to bear weight.

Need for Revision Surgery

Revision means removing or replacing one or more parts of the knee replacement. Reasons can include:

  • infection;
  • loosening or wear;
  • instability;
  • stiffness;
  • fracture;
  • component malposition or malrotation;
  • recurrent kneecap problems; or
  • failure of part of the extensor mechanism.

Most primary knee replacements do not require early revision. The lifetime likelihood depends on factors including age, implant type, reason for surgery and length of follow-up.

Revision surgery is usually more complex than the first knee replacement. It may require larger or more constrained components, stems extending into the bones, bone graft or metal augments. Recovery and results depend heavily on the reason for revision.

An abnormal-looking image does not, by itself, mean revision is required. The decision should combine symptoms, examination, appropriate investigations and a realistic assessment of whether further surgery is likely to help.

Setting Realistic Expectations

A replacement knee is an artificial joint and may not feel identical to a healthy natural knee. Some people remain aware of it during kneeling, stairs or prolonged activity even when pain and mobility are substantially improved.

This does not mean that persistent symptoms should be dismissed. A useful expectation is that surgery aims to produce meaningful improvement in arthritic pain and function—not necessarily complete absence of pain, full movement or a knee that feels entirely forgotten.

Before surgery, consider asking:

  • What improvement is realistic for my current movement and symptoms?
  • What alignment strategy do you plan to use, and why?
  • Will my kneecap be resurfaced?
  • What symptoms could suggest stiffness or instability?
  • When would manipulation under anaesthesia be considered?
  • What findings would justify further imaging or revision surgery?
  • What outcomes does your unit observe after primary knee replacement?

This article provides general information and does not replace individual clinical advice or the consent discussion with your surgeon. New wound problems, fever, rapidly increasing pain or swelling, inability to bear weight, or new loss of movement should be reported promptly using the instructions provided by your hospital.

References

  1. Cheng HY, Beswick AD, Bertram W, et al. What proportion of people have long-term pain after total hip or knee replacement? An update of a systematic review and meta-analysis. BMJ Open. 2025;15. doi:10.1136/bmjopen-2024-088975. Available from: https://pubmed.ncbi.nlm.nih.gov/40398950/
  2. Kahlenberg CA, Nwachukwu BU, McLawhorn AS, Cross MB, Cornell CN, Padgett DE. Patient satisfaction after total knee replacement: a systematic review. HSS J. 2018;14(2):192–201. doi:10.1007/s11420-018-9614-8. Available from: https://pubmed.ncbi.nlm.nih.gov/29983663/
  3. Akhtar M, Razick D, Seibel A, Asad S, Shekhar A, Shelton T. Outcomes of early versus delayed manipulation under anesthesia for stiffness following total knee arthroplasty: a systematic review and meta-analysis. J Arthroplasty. 2024;39(11):2872–2879. doi:10.1016/j.arth.2024.05.059. Available from: https://pubmed.ncbi.nlm.nih.gov/38797451/
  4. Haffar A, Goh GS, Fillingham YA, et al. Treatment of arthrofibrosis and stiffness after total knee arthroplasty: an updated review of the literature. SICOT J. 2022. Available from: https://pubmed.ncbi.nlm.nih.gov/35301559/
  5. Al-Jabri T, Brivio A, Maffulli N, Barrett D. Management of instability after primary total knee arthroplasty: an evidence-based review. J Orthop Surg Res. 2021;16:729. Available from: https://pubmed.ncbi.nlm.nih.gov/34930375/
  6. Larrainzar-Garijo R, et al. Methods of alignment in total knee arthroplasty: a systematic review. Orthopedic Reviews. 2024;16:117769. doi:10.52965/001c.117769. Available from: https://pubmed.ncbi.nlm.nih.gov/38827414/
  7. Saffarini M, et al. Failure modes in malrotated total knee replacement. Knee Surg Sports Traumatol Arthrosc. 2022. Available from: https://pubmed.ncbi.nlm.nih.gov/35920891/
  8. Ebraheim NA, Kelley LH, Liu X, Thomas IS, Steiner RB, Liu J. Periprosthetic fractures about total knee arthroplasty. Orthop Surg. 2020;12(2):322–328. Available from: https://pubmed.ncbi.nlm.nih.gov/31643045/
  9. National Joint Registry. Reports, statistics and patient information. Available from: https://www.njrcentre.org.uk/

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