Analysis of the Patellofemoral Congruence Angle According to the Rotational Alignment of the Femoral Component in Navigation-guided TKA

Abstract

The relative femoral resection plane from the posterior condylar axis was determined by the navigation system. The investigators found that there was a relatively variable range of femoral component rotation intraoperatively (0º-6º) and attempted to determine whether this would affect postoperative patellofemoral congruence. Forty-six varus knees from 34 patients were included in the study; group 1 (15 knees) with 0° or 1° and group 2 (31 knees) with 3º to 6°. The mean (P = .855) and percentage of abnormal values (patellofemoral congruence angle >16°) (P = .193) in preoperative radiographs showed no significant differences between the two groups. In postoperative findings, the mean of patellofemoral congruence angles in group 1 (20.5°) showed a higher tendency than that in group 2 (14.1°), but no statistically significant difference between two groups (P = .089). In conclusion, there was no statistically significant difference in patellofemoral congruence between 2 groups.

Patellofemoral instability is a common problem after total knee arthroplasty (TKA). The incidence of patellofemoral instability after TKA has historically been between 10% and 35%, but with recent improvements in surgical technique, instrumentation, and prosthetic design, these rates have fallen to 1% to 12%.^1-3

Despite advances in surgical technique and implant design, complications involving patellofemoral joint after TKA continue to be the primary cause of pain and the most often cited reason for revision TKA surgery.^1,3,4 Many factors have been implicated as causes of patellofemoral complications after TKA, including preoperative patellar tilt, component design, preparation of the patella, soft tissue balance of patellofemoral joint, surgical approach, and rotational alignment of the femoral component.^1,4

It has been accepted that internal rotation of the component (femur and tibia) is directly related to the severity of patellofemoral complications.^3,4 Even small amounts of internal rotation (1º-4º) are correlated with lateral tracking and patellar tilting.⁴ Higher degrees of internal rotation are related to patellar subluxation and early patellar dislocation.

Navigation-assisted knee surgery has been shown to improve axial alignment and component position.^5,6 Not only can navigation systems monitor sequential ligament release,⁷ they can also measure extension-flexion gaps.⁸ Whereas femoral component rotational position is adjusted by the ligament balancing gap method, these quantitative data can be monitored intraoperatively in real time. In this study, relatively variable range of external rotation was obtained by positioning the femoral component with ligament balancing gap technique using a navigation system. By dividing the patients into 2 groups — 0º or 1º and 3º to 6º — of external rotation, we evaluated the postoperative congruency angle of patellar.

Materials and Methods

Between September 2004 and January 2007, 46 total knee arthroplasty (TKA) procedures were performed in 34 patients by a single surgeon (Y.W.M) with gap technique using the navigation system. Patients included two men (2 knees), and mean age was 66.9 years (range, 51-80). The preoperative diagnosis was osteoarthritis in 44 knees and osteonecrosis in 2 knees. All cases showed varus alignment in preoperative mechanical axis alignments.

Figure 1: Surgical technique resects the proximal tibial surface perpendicular to the mechanical axis of the tibia and resects the posterior condyles to create a rectangular flexion space.

Figure 2: For rectangular flexion gap, various degrees of external rotation (0º-6º) of the femoral component were adopted relative to the posterior condylar axis.

TKA was performed using a navigation system (OrthoPilot; B. Braun Aesculap, Tuttlingen, Germany). This navigation system is an image-free system that uses kinematic analysis of the hip, ankle, and knee joints and anatomic mapping of the knee joint to construct a working model of a patient’s knee.

Implants used in group 1 were E.motion PS (Posterior Stabilizing; B. Braun Aesculap) in 3 cases, e.motion FP (Floating Platform; B. Braun Aesculap) in 4 cases, and E.motion UC (Ultra-Congruency; B. Braun Aesculap) in 8 cases. Implants used in group 2 were e.motion PS in 7 cases, e.motion FP in 15 cases, and E.motion UC in 9 cases.

The surgical approach was a standard median parapatellar incision. The patellar was subluxated laterally and the tibia was subluxated anteriorly. The posterior cruciate ligaments were preserved or sacrificed based on the component used. The medial meniscus and the osteophytes were removed. Soft tissue balancing was carried out before any bone cuts were made. The arrays of the computer navigation system (OrthoPilot) were set up by means of femoral tracker, mounted to a screw. A screw (1 pin) was fixed to the medial aspect of the femur and the tibia, respectively. The position of the selected points required for the system was registered. The center of the proximal tibia was visually identified, and its coordinates were put into the computer by using the pointer specific to the navigation system used. After these localizations, the tibial mechanical axis was determined by a line joining the center of the proximal tibia and the calculated center of the ankle joint. After marking all the reference points, the correction of varus deformity to neutral axis was carried out by medial release in extension position. Limb alignment was checked by navigation system achieving neutral axis. Further soft tissue release and removal of posterior osteophytes were done if necessary at this stage.

Proximal tibial cutting was carried out first at a plane perpendicular to the mechanical axis in the tibia. A laminar spreader, which could adjust the medial and lateral compartments separately, was inserted to identify the adequate collateral tension in extension and 90º of flexion position. These coordinates were recorded, and the femoral cutting block was oriented accordingly to achieve equal extension and flexion gaps (Figure 1). Various degrees of external rotation from 0º to 6º relative to the posterior condylar axis of the femoral component were adopted (Figure 2).

Patellar tracking was tested intraoperatively after implantation by towel clip method. None of the cases showed subluxation or maltracking, and thus the lateral retinacular release was not necessary.

To evaluate, we categorized patients into two groups. In group 1 (15 knees), the adopted external rotation of the femoral component was 0º or 1º and in group 2 (31 knees), the range was 3º to 6º in respect to the posterior condylar axis. Patient demographic data are shown in Table 1, and the number of patients in each femoral external rotation degrees relative to the posterior condylar axis shown by the navigation system intraoperatively is shown on Table 2.

Because the patella was not resurfaced in any case, the postoperative patellofemoral congruence angle could be measured. Patellofemoral congruence angles in this study were measured by Merchant’s view at a fixed angle of 45º in flexed knee in the flexed knee position in pre- and postoperative radiographs (Figure 3). Any congruence angle greater than +16º was defined as abnormal. The angles measured were evaluated by an observer who was blinded to the surgical technique.

Figure 3: Patellofemoral congruence angle. Find the highest point of the medial (B) and lateral (C) condyles and the lowest point of the intercondylar sulcus (A). The angle, BAC, is the sulcus angle. Bisect the sulcus angel to establish the zero reference line, AO. Find the lowest point on the articular ridge of the patella (D). Project line AD. The angle DAO is the congruence angle. All values medial to the zero reference line AO are designated as minus and those lateral as plus.

For comparing pre- and postoperative patellofemoral congruency angles between the two groups, paired the t test and Wilcoxon two-sample test were performed. Values of P < .05 were considered to be statistically significant. Analysis of the data was performed using SAS, version 9.1 (Cary, North Carolina).

Results

The mean preoperative patellofemoral congruence angles were 12.4º (range, 0.13º-47.1º) in group 1 and 9.5º (range, 0.13º-37º) in group 2. Postoperative mean patellofemoral congruence angles in group 1 (20.5º; range, 1.2º-47º) showed a higher values than those in group 2 (14.1º; range, -2.7º-38.4º), but without statistical significance (P = .089) (Figure 4).

The percentages of abnormal preoperative values (patellofemoral congruency angle >16º) were 26.7 % in group 1 and 6.5% in group 2. The mean (P = .855) and percentage of abnormal values (P = .193) in preoperative radiographs showed no significant differences between the two groups. Moreover, the percentage of abnormal postoperative values in each group (66.7% and 41.7%, respectively) showed no statistically significant difference between the 2 groups (P = .116) (Table 3).

Discussion

The stability of the patellofemoral joint is directly affected by several factors, including preoperative patellar tilt; prosthetic component positioning and resultant limb alignment; preparation of the patella; prosthetic design; soft tissue balance; and surgical approach.^3,9,10

The rotational alignment of the femoral and tibial components is one of the most important factors influencing patellofemoral stability because malrotation is one of the most common causes of patellofemoral complications.^11-15 Also, malrotation of the femoral or tibial components has been shown to be a cause of chronic pain with a potential for the patient to develop arthrofibrosis. Specifically for the femoral component, malrotation may lead to patellar instability, ligamentous instability, disturbed functional joint kinematics, and chronic anterior pain.^4,15,16

Figure 4: A, The distribution of preoperative and (B) postoperative patellofemoral congruence angles according to various degrees of external rotation of the femoral component (blue diamonds, group 1; red diamonds, group 2).

The rotational alignment of the femoral component parallel to the transepicondylar axis is known to result in normal patellar tracking, more physiologic ligamentous balance, and minimized patellofemoral shear forces early in flexion.^10,14,17 Thus, most total knee instrument systems in varus or normal alignment have adopted 3º of external rotation relative to the posterior condylar axis to compensate for the nonparallel flexion gap that occurs when the posterior condylar axis is used to determine rotation.

The external rotation of the femoral component is thought to be necessary to compensate for the angular discrepancy that result from the proximal tibial cut perpendicular to the tibial shaft axis because the tibial articular plane is in approximately 3º varus from the tibia shaft axis in normal knees. Insall was the first to describe externally rotating the anterior and posterior femoral resections to correct limb alignment. Rhoads et al² and Anouchi et al¹¹ showed, in studies using anatomic specimen knees, that external rotation of the femoral component makes the patellar tracking close to that of the physiologic knee and significantly improves patellar tracking and reduces patellofemoral complications after TKA. Laskin¹⁸ has introduced the clinical study that the mean degree of external rotation of the posterior femoral resections required to form a rectangular flexion space was 3º to 5º relative to the posterior condylar axis.

In 1998, Berger et al⁴ analyzed the rotational alignment of TKA components in patients with normal mechanical axial alignment by computed tomography (CT) and found that the excessive combined internal rotation of femoral and tibial components correlated directly with the severity of the patellofemoral complication. Small amounts of combined internal rotation (1-4º) correlated with lateral tracking and patellar tilting. Moderate combined internal rotation (3-8º) correlated with patellar subluxation. Large amounts of combined internal rotational (7-17º) correlated with early patellar dislocation or late patellar prosthesis failure. Interestingly, there were no complications in patients with combined external component rotation up to 10º.⁴ Akagi et al^13,14 concluded that the external rotation setting of the femoral component diminished the need for lateral retinacular release and may decrease the rate of patellofemoral complications that occur after TKA. These investigators suggested that setting the femoral component with an external rotation of 3º to 6º relative to the posterior condylar axis is appropriate in a knee with common varus or neutral alignment to set it parallel to the transepicondylar axis.Several studies^2,11,13 showed that additional external rotation of the femoral component from 3º to 10º more than the posterior condylar axis resulted in improved patellofemoral tracking. Miller et al¹⁰ suggest that femoral component rotation parallel to the epicondylar axis resulted in the most normal patellar tracking and minimized patellofemoral shear forces early in flexion.

Several reports^16,19,20 suggest that the transepicondylar axis most consistently recreates a balanced flexion space and normal patellofemoral tracking. Although the ideal reference anatomic axis for femoral component rotation is still in debate, we have chosen the posterior condylar axis as a reference axis. Compared with the transepicondylar axis and Whiteside’s line (anteroposterior line), the posterior condylar line was easily identified intraoperatively and therefore many instruments are designed to align the femoral component in 3 to 5º of external rotation from the posterior condylar line because previous studies have shown that transepicondylar axis is externally rotated from the posterior condylar tangent in 3º to 6º.^14,21,22

In this study, various degrees of external rotation of the femoral component were adopted to achieve rectangular flexion gap from 0º to 6º relative to the posterior condylar line. There is a tendency for the femoral component to be placed in an internally rotated position in varus knees.^7,23 By medial release, and to make rectangular flexion gap simultaneously, the femoral component tends to rotate internally by 0º or 1º. Researchers were curious about whether differences in external rotation of femoral component in respect to posterior condylar line affect the postoperative patellofemoral tracking. By dividing the patients into two groups (group 1, 0º or 1º and group 2, 3-6º of femoral component rotation, although the mean and percentage of abnormal values of patellofemoral congruence angles in group 1 showed a higher tendency than in group 2), there were no statistically significant differences between the two groups.

Managing the ideal axial alignment and correct balancing at the same time are the most difficult aspects of TKA and are primarily dependent on the skill and experience of the surgeon. Although many surgeons are concerned about navigation use as an additional procedure and about increased operation time, instrumentation cost, and additional equipment in operation room, the use of computer technology is no more cumbersome or complicated and provides surgeons with immediate and accurate feedback that are not available with conventional instrumentation. Computer navigation-assisted surgery offers reliable tools to consistently locate important knee and lower limb landmarks, allowing accurate bone cut level and orientation. New versions of navigation systems may also quantify the soft tissue tensional status by use of variable-sized spacer blocks or tensioners. These devices can monitor the soft tissue release sequentially and actually measure the flexion-extension gaps.⁸

There are several limitations to this study. First, the patellofemoral congruence evaluated was based on radiographic measurement on Merchant view at a fixed angle of 45º in the flexed knee. The sensitivity of radiographic assessment of limb and implant alignment may not be as great as that of CT. Computed tomography has been shown to be a more sensitive and accurate method of determining alignment measurements and of assessing component positioning.^4,24 Also, the radiographic measurement on Merchant view at a fixed angle of 45º in the flexed knee does not reflect patellofemoral congruence at all ranges of motion of the patellofemoral joint. Second, although there are several landmarks to determine the femoral component rotation that can be used intraoperatively to ensure correct rotational alignment in TKA,^12,25,26 debate still remains. Olcott and Scott¹⁷ found that flexion space symmetry within 3º of that desired was created with 90% of cases using the transepicondylar axis, 83% using anterior-posterior axis of Whiteside, and 70% using a posterior condylar reference, showing that the posterior condylar line is the least reliable landmark. Third, varus knees tend to have medial condylar wear.^21,23 If femoral component position is adjusted according to this posterior condylar line, the values would be relatively externally rotated. The value of 0º or 1º determined by the navigation system would actually be 3º to 4º of external rotation. This may be the reason for the absence of statistical significance in our results. Fourth, one of the drawbacks of the navigation system is the surgeon’s factor because registration is dependent on the visually identified anatomic landmarks. This leads to errors in the initial mechanical axes and femoral component reference axes formulated by the navigation system. Finally, this study was based on small numbers. More patient cases and longer follow-up are needed to investigate whether the various degrees of external rotation (0º-6º) determined by the navigation system affects the patellofemoral congruency and whether the navigation system has a positive effect on determining optimal rotational alignment of the femoral component in TKA.

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Authors

Drs Moon, Seo, Yang, and Shon are from the Department of Orthopedic Surgery, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea.

Drs Moon, Seo, Yang, and Shon have no relevant financial relationships to disclose.

The authors thank Ms. Min-Jung Lee for her illustrations.

Correspondence should be addressed to: Jae-Hyuk Yang, MD, Department of Orthopaedic Surgery, Samsung Medical Center, Sungkyunkwan University School of Medicine, Ilwon-Dong 50, Kangnam-Gu, Seoul 135-710, Korea.