What complications can occur when the keel hits the posterior cortex during knee replacement?

Keel Impingement on Posterior Tibial Cortex in Knee Replacement

Primary Risk: Periprosthetic Tibial Fracture

When the tibial component keel contacts or penetrates the posterior cortex during knee replacement, the most significant complication is postoperative tibial plateau fracture, which occurs when the posterior keel-cortex distance (KCD) is critically short—particularly less than 3 mm. 1

Mechanism and Fracture Pattern

• Fracture lines characteristically originate between the keel and the posterior cortex, occurring in approximately 80% (12/15) of patients who develop tibial fractures after unicompartmental knee arthroplasty with short posterior KCD 1
• The posterior cortex represents a critical stress point where inadequate bone stock between the keel and cortical wall creates a fracture plane 1
• These fractures typically manifest as periprosthetic fractures, which account for 4.7% of all knee replacement failures requiring revision 2

Critical Distance Thresholds

• Patients with posterior KCD less than 3 mm have significantly higher fracture risk compared to those with adequate distance (mean 2.7 ± 1.6 mm in fracture cases vs 5.2 ± 1.7 mm in non-fracture cases, P < 0.001) 1
• The anterior keel-cortex distance appears less critical, with no documented cases of anterior cortical perforation when APCD measurements were evaluated 1

Contributing Technical Factors

Three specific component malposition errors increase the risk of posterior cortex impingement and subsequent fracture: 1

1. Excessive medial placement of the tibial component (r = 0.30, P < 0.001 correlation with shorter posterior KCD)
2. Distal (low) positioning of the component (r = -0.33, P < 0.001 correlation)
3. Valgus angulation of the tibial tray (r = 0.35, P < 0.001 correlation)

• Patients with any combination of these malpositioning errors demonstrate fracture rates of 18% (7/39) compared to 4.5% (8/178) in properly positioned components (P = 0.008) 1

Anatomical Risk Factors

• Medial overhanging tibial condyle morphology (where the medial eminence line passes medially to the medial tibial cortex) significantly increases fracture risk 1
• Patients with this anatomical variant show fracture rates of 19.6% (10/51) versus 3% (5/166) in normal anatomy (P < 0.001) 1
• These patients inherently have shorter posterior KCD (3.6 ± 1.5 mm vs 5.5 ± 1.8 mm, P < 0.001) 1

Clinical Presentation and Detection

• Many periprosthetic fractures are asymptomatic initially, highlighting the critical importance of routine postoperative radiographic surveillance 2
• Radiographs remain the primary imaging modality for detecting periprosthetic fractures and should include the entire prosthesis with surrounding bone 2
• When radiographs are inconclusive, CT with metal artifact reduction techniques can detect occult fractures 2, 3

Secondary Complications

Beyond immediate fracture risk, posterior cortex impingement can lead to:

• Aseptic loosening (39.9% of all revision cases), as the compromised bone-implant interface may fail over time 2
• Component instability from inadequate bone support 4
• Progressive osteolysis around the compromised fixation 2

Prevention Strategy

Surgeons must maintain minimum 3 mm posterior KCD through: 1

• Preoperative CT-based planning to assess tibial morphology and identify medial overhanging condyles
• Avoiding excessive medial, distal, or valgus positioning during component implantation
• Intraoperative verification of keel position relative to posterior cortex
• Consider alternative implant designs with more laterally positioned posterior pegs in high-risk anatomy 5

Timing and Prognosis

• Patellar fractures (a related complication) typically occur within the first few postoperative years, with incidence up to 5.2% 2
• Periprosthetic fractures represent a serious complication requiring careful evaluation, as they may necessitate revision surgery depending on fracture pattern and component stability 4
• Long-term outcomes depend on early detection and appropriate management, as unrecognized fractures can progress to component loosening and global joint failure 2