Patellofemoral complications are a common cause of pain, morbidity, and complications after total knee arthroplasties (TKA).2-6,18 Alignment and thickness of the resurfaced patellar button and malrotation of the femoral component have been implicated as causes of patellofemoral complications.4,10,17,20,21,27 Femoral component size and position in the anteroposterior (AP) plane also may alter patellofemoral biomechanics, resulting in increased patellar stresses, anterior knee pain, and decreased knee flexion.12,17,21 Posterior referencing commonly is used to accurately match the posterior condylar resection to the posterior thickness of the femoral component, balancing the flexion gap. However, any mismatch between the AP dimension of the host bone and AP dimension of the implant will manifest on the anterior resection and may alter knee kinematics. A femoral implant with an AP dimension less than that of the host bone will lead to notching of the anterior femoral cortex, whereas an implant with an AP dimension greater than that of the host bone may lead to anterior buildup or overstuffing.
The size of the femoral component usually is selected so the anterior femoral resection does not notch the underlying cortical bone. Notching of the anterior cortex decreases axial and torsional load to failure and increases the risk of periprosthetic femoral fractures.14 Culp et al reported a 44% incidence of periprosthetic fractures after notching of the femoral cortex compared with a 1% baseline fracture risk,7 although notching did not increase in the incidence of fracture in another series.23 To prevent notching, a surgeon may overstuff the patellofemoral joint (ie, replace the anterior femoral bone resection with a larger amount of femoral implant). Overstuffing may result in extensor mechanism tightness and subsequent reduction in postoperative knee flexion (Fig 1), but the amount of overstuffing that might decrease postoperative flexion has not yet been ascertained nor has its clinical relevance. However, if a patient can achieve less than 95° flexion it will make functional activities such as rising from a chair and ascending or descending stairs more difficult.22,24,26
Given concerns about the patellofemoral compartment in TKA, it seems an increase in the trochlear groove height would increase the volume of the knee. This may act similarly to an effusion by decreasing the passive and active range of motion (ROM). This overstuffing may occur from an anteriorly displaced or oversized femoral implant versus a well-placed implant in a patient with an atrophic trochlear groove. The same may be true on the patellar surface with resection mismatch creating an anteriorly displaced patellar surface.
We examined whether overstuffing the patellofemoral mechanism in a cadaver model would decrease passive knee flexion and if this mismatch occurred in the clinical setting. We developed a cadaveric model to measure the anterior femoral resection and component thicknesses during TKA to determine if any mismatch may occur in the operating room.
MATERIALS AND METHODS
We determined clinically the amount of overstuffing in the anterior femoral resection after TKA and then used the data to design the experimental study. We first measured the thickness of the anterior resection from 55 patients (mean age, 69 ± 8.6 years) who had primary TKA performed by a fellowship-trained orthopaedic surgeon (WMM). Three implant systems were used: the Duracon® (Stryker Howmedica-Osteonics, Allendale, NJ) was used in 32 patients, the Scorpio® (Stryker Howmedica-Osteonics) was used in 13 patients, and the Profix® (Smith and Nephew, Memphis, TN) was used in 10 patients. Intramedullary (IM) femoral alignment was used to make a 10-mm distal femoral resection at 5° to 7° valgus to the anatomic axis. We used posterior referencing instrumentation to size the AP dimension of the femur. We used anterior referencing as a check to make sure there was no notching of the anterior femur and that the component was not placed too anterior. The implant systems allowed correction of the AP placement of the femoral implant when a size mismatch occurred. The size was selected so the smallest femoral component could be used without notching the anterior femoral cortex. A flat wing fitting into the cutting slot and approximated the anterior resection was used before making any cuts. The femoral component was placed aligned with the AP axis. We used a 1.4-mm thick oscillating cutting blade (Stryker Howmedica-Osteonics) for all resections. The thickness of each anterior resection was compared with the anterior thickness of the implant chosen for each patient. An additional 1.4 mm was added to the anterior resection to compensate for the kerf (thickness) of the blade. The dimensions of the femoral components' lateral anterior flange (h1), medial anterior flange (h3), and trochlear thickness (h2) were obtained directly from the manufacturers (Fig 2), which included the Profix® (Smith and Nephew), Duracon® (Stryker Howmedica-Osteonics), and Scorpio® (Stryker Howmedica-Osteonics) knee systems.
We used analysis of variance (ANOVA) (SPSS, v13.0, Chicago, IL) to compare differences between the anterior resection and anterior implant thickness for each patient between implant groups. Differences were considered significant with a p value < 0.05.
For the experimental study, we obtained 10 fresh frozen cadavers with a mean age of 71 ± 8 years. Whole cadaveric bodies were kept intact to preserve muscle and soft tissue attachments. We used a standard medial parapatellar approach. The patella was everted in all knees without difficulty. An infrared tracker was attached to the femoral diaphysis approximately 10 cm proximal to the joint line, and a tibial marker was attached at the level of the tibial tubercle using a locking threaded Steinmann pin (Stryker Navigation, Kalamazoo, MI). The registration of the lower extremity was performed to represent the lower extremity alignment as described by the manufacturer of the navigation system. The clinical angles between the femur and tibia were represented by calculating the joint coordinate system between the two rigid bodies.11
We then used four towel clips to simulate closure of the knee. The leg was held with the femur in vertical alignment so only the weight of the leg affected knee flexion. We used this method to standardize the measurements between each specimen and trial. Once the baseline passive flexion was measured and recorded for each knee specimen, the height of the medial and lateral trochlear groove was increased with 2-mm and 4-mm thick augments. These augments were made of a pliable aluminum foil molded to the shape of each trochlear groove. Passive flexion was recorded after each augmentation. In addition, 2-mm and 4-mm augments were added to the medial and lateral facets of the patella, and passive knee flexion was measured once again using the navigation system (Fig 2). An ANOVA of repeated measures (SPSS, v13.0) was used to compare differences in passive flexion with and without the augmentation.
The thickness of the lateral flange (h1), medial flange (h3), and trochlear groove (h2) for the femoral components resulted in different heights for all manufacturers. The Scorpio® (Stryker Howmedica-Osteonics) femoral component was the thickest with respect to the anterior medial and lateral flanges and trochlear groove (Table 1).
Intraoperatively, the mean bony resections from the lateral anterior flange, medial anterior flange, and trochlear groove were 7.9 ± 2.5 mm, 5.7 ± 2.3 mm, and 2.8 ± 1.1 mm, respectively. More bone was removed from the lateral flange because of external rotation of the femoral component and anatomic heights of the lateral compared with the medial femoral condyle. The thickness of the lateral anterior flange and medial anterior flange decreased by an average of 1.1 ± 2.6 mm and 0.5 ± 2.2 mm, respectively. The mean change in trochlear groove thickness for all patients and implants was 0.0 ± 1.1 mm. Use of the Duracon® (Stryker Howmedica-Osteonics) and Scorpio® (Stryker Howmedica-Osteonics) components resulted in a mean trochlear groove increase of 0.2 ± 0.7 mm and 0.4 ± 1.7 mm, respectively, compared with a trochlear groove decrease of 0.8 ± 0.7 mm for the Profix® (Smith and Nephew). The Profix® (Smith and Nephew) femoral component resulted in less implants replaced compared with the trochlear groove replacement using the Duracon® (Stryker Howmedica-Osteonics) and Scorpio® (Stryker Howmedica-Osteonics) implants (p = 0.04 and p = 0.001, respectively).
Mean passive flexion for the six cadavers was 132° ± 8.7° before adding any buildup in the anterior compartment. A 2-mm augmentation of the trochlear groove resulted in a 1.3°± 1.2° decrease (p = 0.007) in passive flexion, whereas a 4-mm trochlear groove augmentation decreased (p = 0.001) passive flexion by 4.8° ± 3.2° (Fig 3).
A 2-mm augmentation of the patellar thickness resulted in a 1.8° ± 0.8° decrease (p = 0.017) in passive flexion whereas a 4-mm patellar thickness augmentation decreased (p = 0.009) passive flexion by 4.4° ± 2.5° (Fig 3). Simultaneous 2-mm patellar and 4-mm trochlear groove augmentations decreased (p = 0.043) passive flexion by 5.4° ± 2.4°, which was different from increasing the trochlear groove by 2 mm.
Postoperative range of motion (ROM) is an important outcome measure of TKA and is included in many commonly used scoring systems, such as the Knee Society score and the Western Ontario and McMaster Universities Osteoarthritis (WOMAC) index.21,25 Several variables affecting postoperative ROM include ROM, patellar button thickness, flexion gap, and sacrificing of the posterior cruciate ligament (PCL).1,16,18 Overstuffing the patellofemoral joint also may decrease postoperative knee flexion. Our cadaver study showed anterior buildup on the femur affected passive knee motion. A 4-mm anterior buildup on the femur resulted in approximately 4° loss of passive knee flexion. The same occurred when the augmentation was added to the patellar side of the joint.
As with any cadaveric study there are limitations in the relevance to the in vivo or intraoperative environment. Although there was a difference for the cadaveric model, one could argue whether the 4.8°-average flexion decrease is clinically significant and whether this would be negated by stretch of the capsule and the muscle tension curve of the quadriceps with time. Differences in tissue compliance in vitro and in vivo might render the results of the cadaveric model suspect. Increases in the patellar thickness after TKA has resulted in no differences in postoperative flexion.9,13 Although the peak heights of the lateral and medial aspects of the trochlear groove were compared between the bone resection and the prosthetic implant, volume mismatch may be the more important factor. This could explain some of the reported clinical outliers in flexion.
Although an overstuffed patellofemoral compartment may not be clinically significant, it has been shown to increase the forces and stresses of the patella. In a cadaveric biomechanical model, Oishi et al reported increased shear stresses in the patella after TKA when 2-mm and 4-mm augmentations were added to the patellar button at flexion angles greater than 40°.19 Patellofemoral contact forces also increased after 2-mm augmentations, especially from 70° to 95° flexion.25 It seems prudent to place appropriate-sized components accurately even though it did not affect flexion.
The posterior condyles commonly are referenced for sizing of the femoral component. An advantage of posterior referencing is accurate matching of the posterior bone resection to the posterior thickness of the implant, thereby balancing the flexion gap. However, this may overstuff the patellofemoral joint and decrease knee flexion when a size mismatch occurs between host bone and femoral component. Additional overstuffing may occur if the thickness of the patellar resurfacing button is greater than the patellar resection. We used posterior referencing for femoral component sizing; however, we routinely checked the anterior cortex before making any bone resections. Posterior translation of the femoral component by 1 to 2 mm may be a suitable option to prevent overstuffing, and the femoral component should be slightly flexed.
Using the posterior referencing algorithm and checking the anterior gap resulted in bone resections well matched to the implant thickness regardless of the system or manufacturer. Most manufacturers have less than 5 mm between sizes, which may suggest overstuffing from the anterior resection may never reach more than 4 mm. Anterior referencing was performed by measuring above the trochlear groove. It is the shape of the medial and lateral aspects of the trochlear groove compared with the shape of the femoral implant that causes overstuffing.
The anterior femoral resection is an aspect of TKA not typically measured for a matched bone to implant resection but is dictated by what size implant will not create a notch of the anterior cortex. Although flexion may be affected slightly by patellofemoral overstuffing (∼ 4 mm), the surgeon still should pay attention to the anterior femoral resection and ensure the anterior cut is placed optimally to avoid notching. Anterior and posterior referencing checks can help avoid overstuffing.
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