Knee Replacement Instability: Revision Surgery in Birmingham
Knee replacement instability occurs when the ligaments and soft tissues surrounding the implant fail to provide adequate balanced support, causing the knee to give way or buckle during everyday activities. Mr Shakir Hussain, Consultant Orthopaedic Surgeon at the Royal Orthopaedic Hospital Birmingham, treats knee replacement instability with specialist revision surgery using constrained implant systems.
What is knee replacement instability?
Knee replacement instability occurs when the soft-tissue envelope surrounding a knee arthroplasty fails to provide balanced support in flexion, extension, or both, causing the knee to give way, buckle, or feel unpredictable during activity. It is one of the most common reasons for dissatisfaction after knee replacement and for revision surgery within the first five years of implantation.
A knee replacement functions correctly only when the surrounding ligaments and soft tissues provide balanced tension throughout the range of movement. At primary surgery, the surgeon creates a balanced gap in both flexion (bending) and extension (straightening). If these gaps are not precisely balanced, or if the soft tissues stretch or fail over time, the knee becomes unstable.
Instability is classified by the direction and circumstances in which it occurs. Flexion instability, the most common type, occurs when the knee gives way in mid-flexion (during stair descent, getting into a car, or rising from a chair). Extension instability (recurvatum or hyperextension) is less common and causes the knee to buckle into full straightening. Global instability involves giving way in all planes and is typically associated with severe ligamentous incompetence or multiple failed revisions.
For a patient whose knee replacement was functioning well and has begun to give way, the experience is often frightening and disabling. Understanding the type and cause of instability is essential before selecting the appropriate revision strategy.
What does knee replacement instability feel like?
The knee gives way or buckles unexpectedly, particularly on stairs, slopes, or uneven surfaces. Patients describe a lack of confidence in the joint and often report recurrent swelling after activity. The sensation is different from joint pain: the knee feels unpredictable rather than simply painful.
Giving way or buckling
The most characteristic symptom: the knee suddenly fails to support body weight, causing near-falls or actual falls. This occurs most commonly during stair descent, stepping off a kerb, or on uneven ground.
Difficulty on stairs and slopes
Stair descent requires controlled knee flexion against body weight. When the flexion gap is lax, this movement causes the knee to give way, making stairs disproportionately difficult relative to level walking.
Unpredictable movement
Patients describe a feeling that the knee may move sideways, shift, or fail at any moment. This unpredictability causes anxiety and leads to activity avoidance and reduced independence.
Recurrent swelling
Mechanical instability causes repeated micro-trauma within the joint, producing a synovial reaction with fluid accumulation. The knee swells after activity, particularly after longer walks or climbing stairs.
Pain with specific movements
Pain is often positional rather than constant, triggered by the specific directions of instability. In flexion instability, pain is felt most on bending the knee under load, rather than at rest.
Reduced confidence and withdrawal from activity
Fear of falling causes patients to avoid activities they previously enjoyed, limit walking distance, and avoid public spaces. The psychological impact of instability is often as significant as the physical symptoms.
Why does a knee replacement become unstable?
The most common causes are a mismatch in the flexion and extension gaps created at primary surgery, progressive stretching or failure of the collateral ligaments, and polyethylene spacer wear that allows excessive movement. Patients with rheumatoid arthritis or multiple prior surgeries have a higher background risk of ligamentous incompetence.
The causes of knee replacement instability span surgical technique factors, patient anatomical factors, and implant factors:
- Flexion-extension gap mismatch. At primary surgery, the surgeon creates rectangular spaces in flexion and extension that should be equal in height and filled by the polyethylene spacer. If the flexion gap is larger than the extension gap, the knee is stable in extension but loose in flexion. This is the most common cause of post-operative instability.
- Collateral ligament incompetence. The medial and lateral collateral ligaments resist side-to-side (varus-valgus) movement. If one is damaged at primary surgery, stretched progressively over time, or incompetent pre-operatively due to severe deformity, the knee will be unstable in the coronal plane.
- Polyethylene spacer wear. As the plastic spacer between the metal components wears down over years, the effective joint height is reduced, increasing the relative laxity of the soft tissue envelope and causing progressive instability.
- Soft tissue imbalance at primary surgery. Incomplete release of contracted soft tissues, or over-release of structures that were providing some stability, can leave the knee unbalanced from the time of implantation.
- Post-operative soft tissue trauma. A fall or injury after primary surgery can stretch or disrupt the remaining ligamentous support, precipitating instability in a previously stable replacement.
- Multiple revision surgeries. Each revision episode removes bone and soft tissue, making ligament reconstruction progressively more difficult. Global instability is most common in patients who have undergone multiple revisions.
How is knee replacement instability diagnosed?
Clinical examination with stressed X-rays (taken under valgus and varus stress) demonstrates the degree of ligamentous laxity. Fluoroscopic assessment during movement identifies flexion instability. CT and MRI assess soft tissue integrity. The pattern of instability (flexion, extension, or global) guides the surgical plan.
Accurate diagnosis is the foundation of successful revision. Mr Hussain will take a detailed history of when and under what circumstances the knee gives way, then examine the joint with specific stress tests:
- Varus and valgus stress testing. Applied in both extension and 30 degrees of flexion to assess the medial and lateral collateral ligament integrity. Instability that is greater in extension suggests extensor mechanism or posterior capsule insufficiency; instability predominantly in flexion indicates a flexion gap problem.
- Stressed X-rays. Weight-bearing AP views taken with the knee under valgus and varus stress document the degree of collateral laxity objectively and provide a radiological record of joint space opening.
- Fluoroscopic assessment. Dynamic imaging of the knee during controlled movement (weight-bearing fluoroscopy) identifies paradoxical femoral rollback or anterior tibial subluxation in flexion, confirming flexion gap instability.
- CT scan. Assesses implant rotation, which can be a hidden contributor to instability if the tibial or femoral components are malrotated, creating a gap imbalance in flexion that is not visible on standard X-rays.
- MRI. Assesses soft tissue integrity, including the collateral ligaments, posterior capsule, and quadriceps mechanism, to identify structures that are intact versus compromised.
How is an unstable knee replacement treated?
Mild instability in elderly patients may be managed with a hinged brace, physiotherapy, and activity modification. Significant or progressive instability causing recurrent falls or preventing normal activity requires revision to a more constrained implant system, ranging from a constrained condylar knee (CCK) for moderate instability to a rotating hinge for global instability.
Revision to Constrained Condylar Knee (CCK)
A constrained condylar knee implant has a taller, more conforming polyethylene post and box mechanism that provides medial-lateral stability without fully eliminating rotation. This is the most commonly used implant for flexion instability and moderate collateral laxity. The revision also includes exchange of the bearing surface and often addition of femoral and tibial stems for enhanced fixation.
- Provides medial-lateral constraint without fully restricting rotation
- Taller polyethylene post engages the femoral box in flexion
- Stems added for fixation security in revised bone
- Suitable for flexion instability and moderate collateral laxity
- Preserves more motion than a hinge implant
Revision to Rotating Hinge Implant
A rotating hinge implant mechanically links the femoral and tibial components through a physical hinge, providing stability in all planes regardless of the soft tissue envelope. Rotation is preserved through a rotating platform, which reduces the mechanical stresses on the bone-implant interface. This implant is reserved for global instability, failed CCK revisions, and cases where the collateral ligaments are absent or non-functional.
- Provides full varus-valgus and anteroposterior stability
- Rotating platform reduces bone-implant interface stress
- Long stems required for adequate fixation
- Appropriate when collateral ligaments are non-functional
- Suitable for complex multi-revision cases
Specialist expertise in unstable knee replacement revision in Birmingham
Consultant at the Royal Orthopaedic Hospital
Mr Hussain practises at the Royal Orthopaedic Hospital Birmingham, one of the largest specialist orthopaedic hospitals in Europe, with access to the full range of constrained and hinge revision implant systems.
3,000+ arthroplasty cases from 5,000+ procedures
A high operative volume including complex primary and revision cases. Read more about Mr Hussain's training and background.
MAKO, ROSA, and CORI robotic certifications
Robotic assistance aids in planning soft tissue balancing and component positioning, particularly important in revision cases where altered anatomy makes manual planning challenging.
Complex revision surgery expertise
Mr Hussain performs revision knee replacement for the full spectrum of failure modes. See the dedicated revision surgery page for full details.
Doctify Outstanding Patient Experience 2024, 2025, and 2026
Awarded in three consecutive years, recognising consistently high patient-reported outcomes and communication throughout the surgical journey.
4.98 out of 5 from verified reviews on Doctify. Outstanding Patient Experience Award 2024, 2025, and 2026.
Frequently asked questions about knee replacement instability
For more questions about surgery, recovery, fees, and what to expect, see the full frequently asked questions page or read recent patient testimonials.