Truliant® Femoral Component Fit In Medium To Small Sized Knees

6 years ago

Yifei Dai, PhD
Exactech, Inc.
Weimin Yue, PhD
Exactech, Inc.
Laurent Angibaud, Dipl. Ing.
Exactech, Inc.

INTRODUCTION

In the modern era of total knee arthroplasty (TKA), comprehensive consideration of an individual’s bone and soft tissue characteristics has gained more focus. It is important to select a prosthesis that best fits the native morphology of the targeted patient population. Studies have shown that mediolateral (ML) oversizing of the components can compromise the clinical outcomes of the surgery, such as lower functional scores, less range of flexion, and may account for 27% of postoperative knee pain, possibly due to irritation of the soft-tissue around the knee.^1-3

In the femur, the goal of minimizing component overhang can be complicated by other surgical considerations with unintended consequences. For example, ML overhang can be avoided by undersizing or using a narrower femoral component. In the undersizing circumstance, the undersized anteroposterior (AP) dimension of the component can increase flexion laxity or cause anterior notching. Although these situations may be resolved by additional femoral resections to prepare for the undersized femur, the resulting joint line will inevitably be elevated, which can negatively impact patellofemoral kinematics, increase the incidence of instability, or decrease knee flexion.^4-8 Studies have identified that an overhang of more than 3-4mm is clinically important. In a 2010 study, Mahoney et al. reported that more than 3mm of component overhang can increase the risk of clinically important knee pain by 90%.² A recent investigation by Chung et al. concluded that more than 4mm of overhang can significantly lower the maximum flexion angle postoperatively.⁹

Many studies have evaluated the ML morphological fit of modern femoral implants. Most analyses were focused on the component fit at limited locations, typically at the distal resection area.^2,9,10,11,12However, complex variations in femoral morphology have been reported, which are not limited to the distal portion of the femur.¹³ The design and development of the Exactech’s Truliant® Knee System employed a series of comprehensive morphological studies to minimize ML overhang of the femoral component. This present study computationally assessed the component of the Truliant femoral components superimposed on a dataset of medium to small knees.

Figure 1. Representative views of the Truliant femoral component demonstrating the 10 anatomical locations for the measurement of component fit. — **Figure 1.** *Representative views of the Truliant femoral component demonstrating the 10 anatomical locations for the measurement of component fit.*

MATERIALS AND METHODS

Bone Data
Digital femoral surface models of 30 Chinese (11M/19F) and 24 Caucasian (5M/19F) knees were selected from a CT scan based virtual bone database. The selection included all the right femora in the database that had an AP dimension of no more than 57.9mm measured from the anterior cortex point to the tangent plane of both posterior condyles (corresponding to Truliant component sizes of 3 and under).

Component Sizing and Placement
Each femur was virtually resected in accordance with its proper component size following the Truliant anterior referencing surgical technique (Unigraphics NX, Siemens PLM Software, Plano, TX, USA). Virtual implantation of the femoral component was performed by a computational algorithm. The algorithm first lateralized the femoral component such that it aligned with the bony resection at the AP mid-point of the distal cut (Location 1, Figure 1). The fit of the femoral component was then measured at 10 anatomical locations on the distal, anterior chamfer, and anterior resection areas (Matlab, Mathworks Inc, Natick, MA, USA) (Figure 1). In some instances, if overhang of more than 3mm (clinically important overhang) was detected at any location, a secondary algorithm slightly adjusted the ML position of the component by allowing an ML translation in the opposite direction (no more than 3mm), optimizing the fit by minimizing both the number of locations and the severity of overhang.

Data Analysis

The subsequent ML fit of the component was assessed at the 10 anatomical locations. The incidence of the component extending beyond the resection bony profile were identified. The associated amount of mismatch between the component and bony resection was recorded. Clinically important overhang was defined when the component extended more than 3mm beyond the bony profile.

Figure 2. Illustration of “flattened” two dimensional Truliant femoral component profiles for each component size, overlaid with the associated resected femoral bone profiles in the dataset according to the component sizing and placement. The images were scaled to the same size, with the bone profiles truncated to only the region of interest. — **Figure 2.** Illustration of “flattened” two dimensional Truliant femoral component profiles for each component size, overlaid with the associated resected femoral bone profiles in the dataset according to the component sizing and placement. The images were scaled to the same size, with the bone profiles truncated to only the region of interest.

RESULTS

Figure 2 illustrates the two dimensional unfolded or “flattened” profiles of the Truliant femoral component sizes used in this study (from location 1 to the proximal tip of the flange), overlaid with the associated flattened bony resection profiles according to the component placement. Across genders and ethnicities, the Truliant femoral component design consistently minimized clinically important overhang (Table 1, Figure 2, 3). Only one knee (Chinese, female) had a negligible additional amount (0.1mm) of overhang over 3mm at location 2 lateral.

DISCUSSION

Exactech’s Truliant Knee System continues the successful evolution of the Optetrak Logic® Knee System, with renewed focus on the restoration of patient’s natural anatomy in addition to the clinically proven, patented legacy design for articular and patellar performance.^14-16 The findings of this study showed excellent fit among medium to small sized knees studied in the dataset. Only one knee that had an overhang in excess of the clinically important threshold. Contrary to the common belief that the fit of western originated knee designs may be compromised in other populations, especially Asian patients,^9,10,17 the Truliant femoral design was shown to provide equally good fit for both ethnic groups studied. In addition, although it has been reported that component fit in female knees is inferior to that in male knees,^2,10 the Truliant femoral design did no demonstrate gender based differences in terms of clinically important overhang incidence.

Table 1. Incidence of clinically important overhang (> 3 mm) in the data set. Only one femur had clinically important overhang with an overhang amount of just 0.1 mm above the clinically important overhang threshold. — **Table 1.** Incidence of clinically important overhang (> 3 mm) in the data set. Only one femur had clinically important overhang with an overhang amount of just 0.1 mm above the clinically important overhang threshold.

Figure 3. A representative femur (Chinese, female) with the placement of the Truliant femoral component. No clinically important overhang (> 3mm) was observed on this bone. — **Figure 3.** *A representative femur (Chinese, female) with the placement of the Truliant femoral component. No clinically important overhang (> 3mm) was observed on this bone.*

CONCLUSION

Computational assessment of the femoral fit of Exactech’s Truliant femoral component design demonstrated minimal incidence of clinically important overhang in both gender and ethnic groups investigated. The data confirmed that the sizing of the Truliant femoral component respects the anatomy of the distal femur, potentially minimizing the risk of TKA complications related to femoral component overhang.

References

1. Bonnin MP, Schmidt A, Basiglini L, et al. Mediolateral oversizing influences pain, function, and flexion after TKA. Knee Surg Sports Traumatol Arthrosc. 2013;21:2314–24.
2. Mahoney OM, Kinsey T. Overhang of the femoral component in total knee arthroplasty: risk factors and clinical consequences. J Bone Joint Surg [Am] 2010;92-A:1115–21.
3. Chau R, Gulati A, Pandit H, et al. Tibial component overhang following unicompartmental knee replacement: does it matter? Knee. 2009;16:310–3.
4. Bellemans J. Restoring the joint line in revision TKA: does it matter? Knee. 2004;11:3–5.
5. Yoshii I, Whiteside LA, White SE, et al. Influence of prosthetic joint
line position on knee kinematics and patellar position. J Arthroplasty.
1991;6:169–177.
6. Figgie HE, 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;68:1035–40.
7. Laskin RS. Management of the patella during revision total knee replacement arthroplasty. Orthop Clin North Am. 1998;29:355–60.
8. Singerman R, Davy DT, Goldberg VM. Effects of patella alta and patella infera on patellofemoral contact forces. J Biomech. 1994;27:1059–65.
9. Chung BJ, Kang JY, Kang YG, et al. Clinical implications of femoral anthropometrical features for total knee arthroplasty in Koreans. J Arthroplasty. 2015;30(7):1220-7.
10. Loures FB, de Araújo Góes RF, da Palma IM, et al. Anthropometric study of the knee and its correlation with the size of three implants available for arthroplasty. Revista Brasileira de Ortopedia. 2016;51(3):282-9.
11. Hitt K, Shurman JR II, Greene K, et al. Anthropometric measurements of the human knee: correlation to the sizing of current knee arthroplasty systems. J Bone Joint Surg [Am] 2003;85-A(suppl 4):115–22.
12. Dai Y, Scuderi GR, Penninger C, et al. Increased shape and size offerings of femoral components improve fit during total knee arthroplasty. Knee Surg Sports Traumatol Arthrosc. 2014;22(12):2931-40.
13. Mahfouz M, Abdel Fatah EE, Bowers LS, et al. Three-dimensional morphology of the knee reveals ethnic differences. Clin Orthop Relat Res. 2012;470(1):172-85.
14. Robinson RP, Green TM. Eleven-year implant survival rates of the all polyethylene and metal backed modular Optetrak posterior stabilized knee in bilateral simultaneous cases. J Arthroplasty. 2011;26(8):1165-9.
15. Edwards J, Gradisar I Jr, Nadaud M, et al. Eight and one-half year clinical experience with the Optetrak total knee prosthesis. Presented at the American Academy of Orthopedic Surgeons 2004.
16. Ehrhardt J, Gadinsky N, Lyman S, et al. Average 7-year survivorship and clinical results of a newer primary posterior stabilized total knee arthroplasty. HSS J. 2011;7(2):120-4.
17. Yue B, Varadarajan KM, Ai S, et al. Differences of knee anthropometry between Chinese and white men and women. J Arthroplasty. 2011;26(1):124-30.