Why Your Total Knee Replacement Feels Like It's Giving Way
The sensation of your knee "giving way" after total knee replacement most likely indicates instability, which is the third most common cause of prosthesis failure and requires immediate evaluation with weight-bearing radiographs to assess for component malalignment, loosening, or ligamentous imbalance. 1
Understanding the Problem
Your knee instability represents abnormal and excessive displacement of the prosthetic components, which typically stems from one of three interconnected mechanisms 1:
- Surgical malalignment created during the initial operation, where even minor degrees of instability become progressively worse over time 1
- Component loosening that develops secondary to ongoing instability, creating a vicious cycle where instability leads to malalignment, which then accelerates loosening 1
- Ligamentous imbalance, though this is not the only factor—proper component positioning and alignment are equally critical 1
What You Should Do Immediately
Start with weight-bearing anteroposterior and lateral knee radiographs as your first imaging study. 1 This is the appropriate initial evaluation recommended by the American College of Radiology and can demonstrate:
- Abnormal component alignment or positioning 1
- Periprosthetic lucencies suggesting loosening 1
- Evidence of polyethylene wear 1
- Osteolysis (bone loss around the implant) 1
Additional Imaging if Radiographs Are Inconclusive
If standard radiographs don't reveal the cause, your surgeon should order 2:
- Full-length standing radiographs (hip-to-ankle views) to comprehensively evaluate mechanical axis alignment 2
- CT scanning as the preferred modality for measuring component rotation and precise positioning when malalignment or instability is suspected 2, 3
Timeline Matters for Diagnosis
The timing of your symptoms provides critical diagnostic clues 1:
- Early instability (occurring within the first 4 years on average) usually results from surgical error or poor prosthesis selection 1
- Late instability may indicate progressive loosening from particle-induced osteolysis, where polyethylene wear debris triggers bone loss around the implant 4
Common Pitfall to Avoid
Do not proceed with revision surgery if the cause of instability remains unexplained after thorough evaluation—reoperation without identifying the specific mechanism is associated with suboptimal results. 1 Instability accounts for 7.5% of all revision cases, and the concepts of instability, malalignment, and loosening are so closely interrelated that addressing one without understanding the others leads to failure 1, 2.
What Happens Next
If imaging confirms instability with malalignment or loosening 1, 5:
- Small areas of osteolysis may be monitored with serial imaging 4
- Large areas of osteolysis or significant component loosening typically require revision surgery 4
- Severe instability can progress to frank dislocation if left untreated 1
The key is systematic evaluation before any surgical intervention, as instability represents a specific mode of failure requiring a specific surgical solution 5.