Biomechanical Studies of Stem Fixation in Revision Total Knee Arthroplasty
A cadaveric study by Jazrawi et al compared the effects of stem length, diameter, and mode of fixation on motion and stress transfer of a cemented tibial tray. Twelve fresh cadaveric tibiae had modified tibial tray cemented after resecting the proximal tibia to a depth of 10 mm. Each tibial tray and tibia were instrumented with transformers to measure tray micromotion and strain gauges to measure bone strain at specified levels below the tibial tray. Press-fit stems of 75 × 10 mm, followed by 150 mm stems of increasing diameters, were press fit into the tibiae underreamed by 1 mm. Then, 75- and 150-mm stems were cemented into the tibia. Both types of constructs were tested with both vertical and eccentric loading in varus, valgus, and flexion, and tibial tray motion and bone strain were measured.
This study showed significantly increased tray micromotion with use of short hybrid press-fit stem compared with longer hybrid press-fit stem. Long hybrid press-fit stems achieved tray stability equivalent to short fully cemented stems. A trend for increasing stability of the tibial tray with increasing length and diameter of the hybrid press-fit stem was observed. Completo et al, using strain gauges in cadaveric tibia also demonstrated that the fully cemented stems resulted in more pronounced stress shielding effect on the bone next to the tibial tray compared with the hybrid press-fit stem.
Completo et al also noted increased concentration of strain at the tip of the stem, which was observed with both types of fixation but was more pronounced in the long hybrid press-fit stem. They postulated that this accounted for stem tip pain noted more with hybrid fixation in clinical practice.
The issue of implant stability was also the focus of a recent prospective randomized controlled study by Kosse et al who used radiostereometric analysis to compare the performance of 12 fully cemented and 11 hybrid stem fixations at a mean 6.5-year follow-up. They did not observe any statistically significant difference between both modes of stem fixation. However, they showed that 5 cemented tibial stems had more than 1-mm micromotion, which might increase the risk of mid-term/long-term loosening, compared with none in the hybrid group. This was, however, not accompanied by radiolucent lines around the implants or the presence of clinical symptoms.
An advantage of diaphyseal engaging stem fixation compared with short fully cemented stem is an accurate mechanical alignment, a prerequisite for successful long-term knee function after knee arthroplasty.
An important variable in achieving accurate mechanical alignment is the canal fill ratio. The canal fill ratio is the ratio formed by dividing the diameter of the stem measured at 2 and 7 cm from the tip of the stem by the width of the diaphyseal medullary canal at those levels. A canal fill ratio greater than 85% is associated with restoration of normal mechanical alignment.
Completo et al showed that use of titanium rather than cobalt chrome resulted in lower strain at the stem tip. A clinical study by Barack et al corroborated these biomechanical findings by showing significantly reduced incidence of stem tip pain in a slotted titanium stem (8.1%) when compared with a solid cobalt chrome stem (18.8%).
J Am Acad Orthop Surg. 2022;30(9):e703-e713. © 2022 American Academy of Orthopaedic Surgeons