Joint Line Changes After Navigation-assisted Mobile-bearing TKA

Abstract

The purpose of this study was to analyze the clinical results of patients who underwent navigation-assisted cruciate ligament retention-type mobile-bearing total knee arthroplasty (TKA) according to joint line changes. From September 2004 to January 2006, cruciate ligament retention-type mobile-bearing TKAs were performed using a navigation system (OrthoPilot; B. Braun Aesculap, Tuttlingen, Germany) on 50 knees in 45 patients (2 men, 43 women). The mean follow-up period was 46 (range, 39-55 months), and patient mean age was 65 years. There was 1 case of rheumatic arthritis; all others were of degenerative arthritis. Proximal tibia resection was performed at the sclerotic level of the medial tibial plateau. The distance from the lowest point of lateral tibial plateau (registered point) to the proximal resection plane was measured. Clinical outcomes (range of motion, Knee Society Score) were compared for joint line elevations of <3 mm (20 cases) and <3 mm (30 cases). Mean joint line elevation was 1.93 mm (range, –1-5 mm). Joint line change was not found to be associated with difference of clinical results (P>.05). The findings of this study suggest that joint line changes in the range of –1 to 5 mm after cruciate ligament retention-type mobile-bearing TKA do not affect clinical outcome.

The accurate restoration of the joint line and the achievement of balanced flexion and extension gaps are prerequisites of successful total knee arthroplasty (TKA).

Research has shown that functional results of replaced knees can be significantly compromised if the joint line is elevated or lowered from its natural position. Furthermore, cadaver studies have associated a decrease in postoperative flexion, an increase in patellofemoral forces (potentially resulting in subluxation, dislocation, or fracture),^1,2 and an increase in varus-valgus laxity³ with as little as a 5-mm shift in the position of the joint line. Clinical results have also shown that joint line position is related to postoperative functional results after TKAs (primary or revision). In an analysis of 116 total knees, Figgie et al⁴ found that results were improved when the joint line was located within 8 mm of its normal preoperative position and was placed posteriorly enough to allow restoration of the extensor moment arm. If the anatomic joint line is moved proximally, the resultant patella baja can cause impingement of the patellar implant on the front of the tibial component, which presents clinically as persistent anterior knee pain and decreased flexion.⁵ Conversely, patella alta, which increases patellar strains, alters extensor mechanism tracking, and increases the risk of subluxation,¹ is often caused when the anatomic joint line is moved distally.

Mobile-bearing TKAs require precise balancing in both extension and flexion gaps; therefore, any significant alteration in joint line position might be assumed to be detrimental to clinical outcome. Computer-assisted surgery was introduced to improve TKA alignment by improving joint line position. Relatively few studies have examined the role of the joint line on final outcome after TKA, and most of these studies have concentrated on TKA revision.^6,7 The restoration of joint line in mobile-bearing TKAs has received very little research attention,⁸ and our literature search failed to unearth any report that mentions joint line changes after navigation-assisted surgery. In this prospective study, we investigated the influence of joint line position on pain, function, and range of motion after posterior cruciate-retaining mobile-bearing TKA.

Materials and Methods

In this prospective case study, we investigated 50 total knee prostheses in 45 patients. All operations were performed by a single surgeon (Y.W.M.) between September 2004 and January 2006. The cohort was composed of 43 women and 2 men with an average age of 65 years. The mean follow-up period was 46 months (range, 39-55 months). There was 1 case of rheumatic arthritis, but all other cases were of degenerative arthritis. In addition, 26 cases (52%) underwent patella resurfacing.

Total knee arthroplasty was performed using the OrthoPilot navigation system (B. Braun Aesculap, Tuttlingen, Germany). This image-free system uses kinematic analysis of hip, ankle, and knee joints and anatomic mapping of the knee joint to construct a working model of the patient’s knee. A cruciate ligament retention-type mobile-bearing floating platform (E.motion, B. Braun Aesculap) was implanted in all patients. Patients were divided into 2 groups: those with a joint line change of >3 mm or greater and those with joint line change < 3 mm (Table 1).

Table 1: Patient Demographics

Each patient received routine preoperative and postoperative clinical care. Evaluations included serial radiographs, knee range of motion, and all components of the Knee Society Score (KSS) system.⁹

Operation Technique

In all cases, the surgical approach was via standard median parapatellar incision. The patella was subluxed laterally, the tibia was subluxed anteriorly, and the posterior cruciate ligaments were preserved. The medial meniscus and osteophytes were removed, and soft tissue balancing was conducted before any bone cuts were made. The arrays of the OrthoPilot navigation system were arranged using femoral trackers mounted to screws. A screw was fixed to the medial aspect of the femur and the tibia. At the registration stage, the positions of selected points, including the lowest point of the lateral tibial plateau, were registered. The center of the proximal tibia was visually identified and its coordinates were input into the computer using a pointer specific to the navigation system. The tibial mechanical axis was then defined as the line joining the center of the proximal tibia and the calculated center of the ankle joint. After marking all reference points, correction of varus deformity with respect to the neutral axis was carried out by medial release in extension. Limb alignment was checked using the navigation system to achieve a neutral axis. Further soft tissue release and posterior osteophyte removal were done, if necessary, at this stage.

Proximal tibial cutting was done in a perpendicular plane to the mechanical axis of the tibia. The authors intended to incorporate the resection plane at the sclerotic level of the medial tibial plateau with 2° of posterior slope. The thickness of tibial bone about to be resected was displayed on a monitor in real time (Figure 1). After resection of the proximal tibia, actual tibial thickness and slope were displayed on the monitor and identified using the “verifier” (Figure 2). Joint line values were calculated by subtracting the thickness of resected tibial bone from the thickness of the polyethylene insert. Values were considered positive if the joint line moved proximally.

Figure 1: The thickness of tibial bone ready for resection is displayed on a monitor in real time.

Figure 2: Further bone cutting was performed to strictly match values measured before bone cutting

Figure 2: After resecting the proximal tibia, actual tibial thickness and slope are displayed on the monitor identified using the "verifier." Further bone cutting was performed to strictly match values measured before bone cutting.

A laminar spreader that could separately adjust the medial and lateral compartments was inserted to determine adequate collateral tension in extension and at 90° of flexion. These coordinates were then recorded and the femoral cutting block was oriented to achieve equal extension and flexion gaps. Sometimes thicker inserts were needed to match the gap. Implantation was performed with bone cement in all cases. Patellar tracking was tested intraoperatively using the towel clip method, and lateral retinacular release was performed when necessary.

Statistical Analysis

Comparisons between means of the variable of interest were performed using the paired t test and the Wilcoxon two-sample test after checking normality using the Kolmogorov-Smirnov test. Means, standard deviations (SDs), and 95% confidence intervals (CIs) were calculated. Data analysis was performed using Excel 2003 (Microsoft Corp, Redmond, Washington) and the Statistical Analysis System (Ver 9.1; SAS Institute, Inc., Cary, North Carolina). P<.05 was considered statistically significant.

Results

A total of 50 primary TKAs were performed in 45 patients. All patients were followed up for a mean 46 months (range, 39-55 months). Of the 50 cases, 20 cases (40%) showed a joint line elevation of >3 mm, whereas in 30 cases (60%), joint line was restored to <3 mm (Table 2). The intended polyethylene insert thickness was 10 mm in all cases. During gap balancing, readjustment of insert size was done, and a 12-mm polyethylene insert was used in 16 cases (32%). There were no postoperative instability, infection, or patellar tracking problems.

Table 2: Number of Cases According to Joint Line Changes

Average preoperative flexion in group I (joint line elevation >3 mm) was 123° (range, 110°-140°), with an average flexion contracture deformity of 9.68° (range, 0°-30°), whereas average preoperative flexion in group II (joint line elevation <3 mm) was 121° (range, 95°-140°), with an average flexion contracture deformity of 9.33° (range, 0°-25°). The average postoperative flexion in group I was 120° (range, 105°-140°), with an average flexion contracture deformity of 2.8° (range, 0°-10°), and average postoperative flexion for group II was 124° (range, 110°-140°), with an average flexion contracture deformity of 3.1° (range, 0°-10°). The change of range of motion from preoperative to postoperative state was not significant between the 2 groups (P=.06; P>.05).

In group I, average preoperative clinical KSS was 49.6 (range, 21-80) and average preoperative functional KSS was 58.3 (range, 30-70). In group II, these scores were 47.19 (range, 28-74) and 57.97 (range, 10-80), respectively. Postoperatively in group I, the average clinical KSS was 94.2 (range, 89-100) and average functional KSS was 92.33 (range, 80-100). Postoperatively in group II, these scores were 93.19 (range, 88-100) and 93.92 (range, 82-100), respectively (Table 1). Although the changes in clinical and functional KSS scores from the preoperative to the postoperative state were significant (P<.0001), no significant group difference was observed (P>.05).

Discussion

Alteration of the joint line can have a significant effect on flexion and extension and on the extensor mechanism. A review of the literature suggested that TKA outcome is related to joint level and that elevation of the joint line by 5 to 8 mm or more can have a detrimental effect on final results.⁷ However, the majority of published studies^6,7,10-14 examined the role of joint line in revision cases, although recently Selvarajah and Hooper⁸ published clinical results of primary TKA performed using a cruciate-retaining type mobile-bearing arthroplasty. In the study, joint line change averaged +1.1 mm (SD=4.1 mm; P=.017), and no correlation was found between joint line elevation in the range of -11 to +10 mm and patellar position, knee pain or function scores, or range of motion (P>.05).

In our study, joint line measurements were performed using a navigation system intraoperatively to provide details of joint line changes in real time. However, outcome variables were found to be similar for the 2 joint line groups (joint line elevation >3 mm and joint line restoration within 3 mm). Accordingly, our results suggest that joint line changes in the range of -1 to 5 mm after cruciate ligament retention-type mobile-bearing TKA result in satisfactory clinical outcomes.

A navigation system was used in all cases in our study. Although image-free navigation systems are criticized for increasing operation times and costs, and for adding to space constraints in operating rooms, we do not find this computer technology to be more cumbersome or complicated, and it has the benefit of providing immediate and accurate feedback, which is not possible when conventional instrumentation is used. Furthermore, computer-assisted navigation surgery provides reliable tools that consistently locate important knee and lower limb landmarks, and allows bone cuts to be accurately placed at correct levels and orientations.^15,16 Although navigation systems are dependent upon the registration process, we believe that this accuracy of navigation is a strong aspect of this study. Furthermore, all procedures were performed by a single surgeon (Y.W.M.) and the study was designed prospectively.

There is no standard anatomic radiographic measuring system that can correctly identify the joint line. Some surgeons measure from the adductor tubercle to the joint line of the distal femur, whereas others measure from the lateral flare of the distal femur to the joint line. Some measure in anteroposterior (AP) view, whereas others measure in lateral view from the posterior flare of the femoral condyle distal to the joint line, from the tip of the fibular head proximal to the proximal tibial surface, or from the tibial tubercle proximal to the joint surface.¹¹ Methods based on the use of the fibular head as reference point for the joint line appear difficult, because in many cases the tibial component obscures the fibular head after implantation (in AP or mediolateral view). Those based on the use of the distal femur as a reference point for joint line measurement also appear difficult, because they require a weight-bearing radiographic view with the femoral condyles overlapping, which is not always available. The second most common method, especially during revisional surgery, involves using anatomic landmarks intraoperatively as a guide to joint line level. Bony landmarks include the epicondyles, fibular head, and tibial tuberosity. However, these methods have been criticized in terms of consistency and reproducibility. The method used to determine joint line in our study involved simply subtracting resected bone thickness from the thickness of the polyethylene insert used. The lowest point of the lateral tibia plateau was registered in the navigation system, and the thickness of tibial bone about to be resected was presented on a monitor in real time during proximal tibia resection guide application.

The surgical goals and techniques (alignment, bone resections, and ligamentous balancing) required for implanting a mobile-bearing TKA are theoretically no different from those used for fixed-bearing TKA. Moreover, soft tissue balancing, the creation of equal flexion and extension gaps, and precise component positioning are extremely important for both fixed and mobile-bearing TKA systems. However, extension and flexion gap balance is of particular importance during mobile-bearing TKA, because imbalance increases the risks of bearing dislocation or spinout, in which the polyethylene bearing is no longer congruous with the femoral component. Gap balance can be achieved by several methods. In our experience, some types of tensioning instruments (eg, laminar spreaders, spacer blocks, specific gap tensioning devices) provide the most reliable and reproducible means of achieving extension and flexion gap balance and tension. Specific gap-tensioning devices provide the additional advantage of facilitating flexion gap width equalization to previously established extension gaps. These tensioning devices allow measurements (width and tension) of a balanced extension gap to determine direct flexion gap bone resection and femoral component rotation. When balancing disproportionate spaces, we sometimes implanted an oversized insert (a 12-mm polyethylene insert in 16 of 50 cases), which could account for the elevated mean joint line of 1.93 mm obtained in this study.

Conclusion

There are no standard intraoperative or anatomic radiographic measuring systems that can correctly identify the joint line. Of the various methods used to identify the joint line, image-free navigation provides the resection plane values in real time. The method used in this study to determine the joint line involved subtracting the resected proximal tibia bone thickness from the thickness of the polyethylene insert. The clinical results of the group with joint line elevation of >3 mm did not differ significantly from those with joint line elevation of <3 mm in cruciate-retaining TKA. Although the navigation-assisted method for identifying the joint line may not be an accurate method, it may be one of the tools used intraoperatively.

References

Singerman R, Heiple KG, Davy DT, Goldberg VM. Effect of tibial component position on patellar strain following total knee arthroplasty. J Arthroplasty. 1995; 10(5):651-656.
Singerman R, Davy DT, Goldberg VM. Effects of patella alta and patella infera on patellofemoral contact forces. J Biomech. 1994; 27(8):1059-1065.
Martin JW, Whiteside LA. The influence of joint line position on knee stability after condylar knee arthroplasty. Clin Orthop Relat Res. 1990; (259):146-156.
Figgie HE 3rd, Goldberg VM, Heiple KG, et al. The influence of tibial-patellofemoral location on function of the knee in patients with the posterior stabilized condylar knee prosthesis. J Bone Joint Surg Am. 1986; 68(7):1035-1040.
Yoshii I, Whiteside LA, White SE, Milliano MT. Influence of prosthetic joint line position on knee kinematics and patellar position. J Arthroplasty. 1991; 6(2):169-177.
Laskin RS. Joint line position restoration during revision total knee replacement. Clin Orthop Relat Res. 2002; (404):169-171.
Partington PF, Sawhney J, Rorabeck CH, et al. Joint line restoration after revision total knee arthroplasty. Clin Orthop Relat Res. 1999; (367):165-171.
Selvarajah E, Hooper G. Restoration of the joint line in total knee arthroplasty. J Arthroplasty. Aug 29, 2008 [Epub ahead of print].
Insall JN, Dorr LD, Scott RD, Scott WN. Rationale of the Knee Society clinical rating system. Clin Orthop Relat Res. 1989; (248):13-14.
Bellemans J. Restoring the joint line in revision TKA: does it matter? Knee. 2004; 11(1):3-5.
Hoeffel DP, Rubash HE. Revision total knee arthroplasty: current rationale and techniques for femoral component revision. Clin Orthop Relat Res. 2000; (380):116-132.
ten Ham AM, Wymenga AB, Jacobs WC. The use of the knee joint-line balancer to control patella position in revision total knee arthroplasty. Knee. 2005; 12(2):89-92.
Mason M, Belisle A, Bonutti P, et al. An accurate and reproducible method for locating the joint line during a revision total knee arthroplasty. J Arthroplasty. 2006; 21(8):1147-1153.
Porteous AJ, Hassaballa MA, Newman JH. Does the joint line matter in revision total knee replacement? J Bone Joint Surg Br. 2008; 90(7):879-884.
Bathis H, Perlick L, Tingart M, et al. Alignment in total knee arthroplasty. A comparison of computer-assisted surgery with the conventional technique. J Bone Joint Surg Br. 2004; 86(5):682-687.
Chauhan SK, Scott RG, Breidahl W, Beaver RJ. Computer-assisted knee arthroplasty versus a conventional jig-based technique. A randomised, prospective trial. J Bone Joint Surg Br. 2004; 86(3):372-377.

Authors

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

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

Correspondence should be addressed to: Young-Wan Moon, MD, Department of Orthopedic Surgery, Samsung Medical Center, Sungkyunkwan University School of Medicine, 135-710, Ilwon-dong 50, Kangnam-Gu, Seoul, Korea.

doi: 10.3928/01477447-20090915-57