How Can Surgeons Prevent and Treat Instability in TKA Patients?
Dr. William Barrett answers questions from ICJR about the types of knee instability can develop in total knee arthroplasty patients, the steps surgeons can take to prevent instability, the workup for suspected instability, and the treatment options available.
ICJR: How common is knee instability following total knee arthroplasty (TKA), and why does it occur?
William P. Barrett, MD: TKA instability accounts for approximately 25% of knee revisions. [1-3] Lesser degrees of instability can contribute to the 20% of patients who are dissatisfied with the results of their TKA.
Instability can be classified as extension instability or flexion instability:
- Extension instability is either symmetric, due to excess femoral bone removal, or asymmetric, resulting from residual angular deformity or failure to balance the soft tissues.
- Flexion instability is caused by unbalanced flexion/extension gaps. Most commonly, this results from excess posterior femoral condylar bone excision (Figure 1), excess tibial posterior slope (Figure 2), or a combination of both. Other causes include iatrogenic damage to the medial collateral ligament (MCL) or traumatic injury to the MCL.
Figure 1. Excess posterior femoral resection leading to decreased posterior condylar offset.
Figure 2. Excess posterior tibial slope causing a lax flexion gap.
ICJR: What can the surgeon do to prevent knee instability?
Dr. Barrett: Prevention begins with appropriate preoperative planning to correct existing angular deformity and anticipate the soft tissue releases that will be needed. During the procedure, the surgeon should be aware of the following:
- Ensure adequate resection by accurately placing the femoral and tibial cutting blocks and measuring the bone to be resected.
- Ensure that the flexion and extension gaps are equal with either measured resection or gap balancing techniques.
- Minimize the risk of flexion instability by avoiding excess posterior slope with either cruciate retaining or substituting knee designs.
- Minimize the risk of iatrogenic MCL damage with careful retractor placement when making the medial posterior femoral condylar cut and proximal tibial cut (Figure 3).
- If the MCL is cut during TKA, repairing and bracing the knee can result in satisfactory outcomes. 
Figure 3. Retractor placement for protecting the medial collateral ligament.
ICJR: What is the typical workup of a patient who presents with suspected instability post–TKA?
Dr. Barrett: Extension instability is fairly straightforward to diagnose, as the knee is either malaligned or unstable to varus/valgus stress in extension.
Flexion instability is more challenging. Patients can present with a complaint that their knee “just doesn’t feel right.” They will give a history of difficulty going up and down stairs and/or trouble getting in and out of chairs. They will often have anterior knee pain and swelling due to anterior translation of the tibia stretching the anterior soft tissue of the knee.
On exam, these patients will have an effusion, good range of motion, tenderness over the pes insertion, and anterior-posterior (AP) translation greater than 1 cm with the knee relaxed at 90° of flexion (Figure 4). This is diagnostic of flexion instability. Radiographs of the knee are often normal but can reveal decreased posterior condylar offset. Loaded flexed knee lateral radiographs can demonstrate abnormal AP translation (Figure 5).
Figure 4. More than 1 cm of anterior-posterior translation = flexion instability.
Figure 5. Lateral radiograph demonstrating increased anterior-posterior laxity.
Patients with flexion instability often present with some muscle atrophy, so a course of physical therapy can be helpful in mild instability. A course of functional bracing can also be helpful. If bracing provides symptomatic relief, then surgical correction of the flexion instability can provide similar functional improvement.
ICJR: What are the implant and surgical options to stabilize the knee and how do you decide which ones to use?
Dr. Barrett: The intervention depends on the type of instability:
- Loose in flexion: Upsize the femoral component to increase the posterior condylar offset; translate the femoral component posteriorly; reduce the posterior tibial slope
- Loose in extension: Move the femoral component distally
- Loose in flexion and extension: Use a thicker tibial insert
- Cannot balance gaps: Use a constrained implant
With the most common form of instability – flexion instability – the key is to diagnose what is wrong and correct it. The flexion space is lax and is the result of excess posterior femoral resection or excess posterior tibial slope. A stepwise correction was outlined by Abdel et al. 
Upsizing the femoral component and translating it posteriorly using posterior femoral augments will increase the posterior condylar offset. On the tibial side, decreasing the posterior slope will also tighten the flexion gap (Figure 6).
Figure 6. Correction of flexion instability by upsizing the femoral component, translating it posteriorly to increase the posterior condylar offset, and using posterior femoral augments.
The use of posterior stabilized implants is usually adequate if the gaps can be corrected. In more severe cases, a varus/valgus constrained device may be necessary. In extreme cases, a rotating hinge can be a salvage option. In any case, revision of both components is preferred. The results of revision for flexion instability show improvement, but generally not as much as revision for loosening or infections. 
- Diagnosis can be challenging
- Exclude other causes of pain
- Prevention with good surgical technique
- Determine the cause of instability
- Revise both components in the majority of cases
- May need to increase implant constraint if gaps are not balanced
- Results are not as predictable compared with loosening/sepsis
William P. Barrett, MD, is from Proliance Orthopedic Associates in Renton, Washington.
Disclosures: Dr. Barrett has disclosed that he receives royalties and research support from and is a consultant for DePuy Synthes.
- Fehring TK, Odum S, Griffin WL, Mason JB, Nadaud M. Early failures in total knee arthroplasty. Clin Orthop Relat Res 2001:315e
- Sharkey PF, Hozack WJ, Rothman RH, Shastri S, Jacoby SM. Insall Award paper. Why are total knee arthroplasties failing today? Clin Orthop Relat Res 2002
- Pitta M, Esposito CI, Li Z, Lee YY, Wright TM, Padgett DE. Failure after modern total knee Arthroplasty: A prospective study of 18,065 knees. J. Arthroplasty 33 2018 407-414
- Bohl DD, Wetters NG, Del Gazio DJ, Jacobs JJ, Rosenberg AG, Della Valle CJ. Repair of intraoperative injury to the medial collateral ligament during primary TKA JBJS 2016
- Abdel MP, Pulido L, Severson EP, Hanssen AD. Stepwise surgical correction of instability in flexion after total knee replacement. JBJS-B 2014
- Grayson CW, Warth LC, Ziemba-Davis MM, Meneghini RM. Functional improvement and Expectations are diminished in TKA patients revised for flexion instability compared to aseptic loosening and infection. J. Arthroplasty 31 (2016) 2241-2246