Knee replacements are common orthopedic surgical procedures performed to relieve pain and restore function in knees damaged by arthritis, injury, or other conditions. Over 600,000 knee replacements are performed in the United States each year (1). During the procedure, damaged cartilage and bone is removed and replaced with artificial components made of metal alloys, plastics, and ceramics to recreate the function of a normal knee joint. Understanding the average weight of the components used in knee replacement surgery can provide insight into the materials, design, and success rates of these devices. This article reviews research on the typical weights of the femoral, tibial, patellar, and polyethylene components in total and partial knee replacement systems.
Average Component Weights in Total Knee Replacements
Total knee replacement (TKR) involves replacement of all three compartments of the knee – the medial, lateral, and patellofemoral joints. TKR utilizes four main components (2):
- Femoral component – caps the end of the femur bone and articulates with the tibial component.
- Tibial component – attaches to the top of the tibia bone and articulates with the femoral component.
- Patellar component – resurfaces the back of the kneecap (patella).
- Polyethylene insert – sits between the tibial and femoral components to provide a smooth gliding surface.
Research on the typical weight of each of these components has found:
Femoral Component
The femoral component accounts for the largest portion of the implant construct weight, averaging around 120 g (3–6). However, weight can vary depending on design and fixation method:
- Cemented components tend to be lighter than cementless components. One study found cemented femoral components weighed an average of 108 g compared to 129 g for cementless (7).
- Gender-specific components designed for female anatomy are lighter, with one brand reporting weights of 110 g vs. 130 g for gender-neutral (8).
- Oxidized zirconium femoral components may weigh slightly more than cobalt chromium at approximately 130 g (9).
Overall, femoral component weight typically ranges from 100-140 g.
Tibial Component
The tibial baseplate usually weighs 60-80 g depending on fixation method (3–6):
- Cemented baseplates average around 60-70 g.
- Cementless baseplates are heavier at approximately 80 g.
Patellar Component
Patellar components are the lightest of the knee replacement parts, weighing approximately 30-40 g depending on design (3–6,10):
- All-polyethylene patellar components average around 30 g.
- Metal-backed patellas are slightly heavier at approximately 40 g.
Polyethylene Insert
The ultra high molecular weight polyethylene (UHMWPE) insert averages between 60-80 g (3–6,10):
- All-polyethylene tibial components with no metal baseplate may use a heavier polyethylene insert around 80 g.
- Inserts for two-piece modular tibial components are often lighter in the 60-70 g range.
In total, the average weight of a TKR implant is approximately 300 g (range 270-330 g) (10). However, this can vary between different implant brands and component configurations.
Component Weights in Partial Knee Replacements
In contrast to TKR, partial knee replacement (PKR) only resurfaces one or two compartments of the knee. PKR involves implantation of the following:
- Femoral component
- Tibial component
- May include patellar component
Since the polyethylene insert is smaller in PKR, the overall implant weight is less than TKR. Reported weights include:
- Medial PKR: 220 g (range 190-240 g) (11)
- Lateral PKR: 190 g (range 170-220 g) (11)
- Patellofemoral PKR: 140 g (range 130-170 g) (11)
The components utilize similar materials and fixation methods to TKR implants, so have comparable individual weights by part. The reduced overall weight reflects the smaller sizes and fewer components implanted.
Factors Affecting Knee Replacement Component Weights
Several factors can impact the weight of knee replacement components, including:
Patient Sex and Size
Larger femoral and tibial components designed for male patients tend to be heavier than smaller sizes suited for females (7,8). Component weight also scales with patient BMI and the corresponding implant size required.
Implant Material
Common materials like cobalt chrome, titanium, and oxidized zirconium have differing densities that affect part weights. Use of porous metals and hydroxyapatite coatings on cementless components also increases weight compared to solid castings.
Component Fixation
Cementless parts designed for bone ingrowth typically incorporate geometric undercuts, textured surfaces, and other features that make them heavier than cemented components (7).
Modular vs. Monoblock Designs
Monolithic component designs are generally lighter than modular components of equivalent size. However, modularity provides flexibility and adaptability for accommodating various patient requirements.
Sterilization Method
Gamma irradiation in air versus inert environments produces subtle material property changes that can impact weight measurements (12).
Significance of Knee Replacement Weight
The weight of knee replacement components has a few implications on surgical outcomes and device performance:
Surgical Ease and Precision
Heavier components can make delicate implant positioning and alignment more challenging (8). Lightweight designs enhance surgical handling.
Bone Remodeling and Stress Shielding
Excessive implant weight alters the normal biomechanics of the knee, which can lead to risk of prosthetic loosening, migration, and failure (13). Lighter components help minimize these complications.
Impact Forces and Wear
Heavy components increase contact stresses against the polyethylene insert, resulting in more rapid wear and debris generation (14). Optimizing component weight distributes forces favorably to enhance longevity.
Biomechanical Function
Overly light designs may not provide adequate joint stability or load-bearing support, while heavy constructs restrict natural motion. An appropriate weight balance is important for normal knee mechanics.
Patient Satisfaction
Lower extremity sensation can be affected by variations in implant weight and mass. Patient comfort andFeedback have demonstrated preferences for lighter weight knee designs (8).
Conclusion
The typical weight of a total knee replacement is approximately 300 g, while partial knee replacements are lighter ranging from 140-220 g depending on the components used. The femoral and tibial parts generally make up the bulk of the weight at around 100-140 g and 60-80 g respectively, with the patellar and polyethylene insert contributing approximately 30-40 g and 60-80 g each. However, many factors can influence knee replacement component weights, including patient characteristics, implant materials and design, fixation method, modularity, and sterilization technique. Optimizing the weight of knee replacement components improves surgical handling, biomechanics, wear performance, and patient satisfaction. Continued advances in orthopedic biomaterials and implant engineering aim to strike an ideal weight balance for maximizing knee replacement outcomes and longevity.
| Component | Average Weight Range |
|---|---|
| Femoral | 100-140 g |
| Tibial Baseplate | 60-80 g |
| Patellar | 30-40 g |
| Polyethylene Insert | 60-80 g |
| Total Knee Replacement | 270-330 g |
| Partial Knee Replacement | Average Weight Range |
|---|---|
| Medial | 190-240 g |
| Lateral | 170-220 g |
| Patellofemoral | 130-170 g |
References
1. American Academy of Orthopaedic Surgeons. (2015). Total knee replacement. OrthoInfo. https://orthoinfo.aaos.org/en/treatment/total-knee-replacement/
2. Siqueira, M. B., Klika, A. K., Higuera, C. A., & Barsoum, W. K. (2015). Modes of failure of total knee arthroplasty: registries and realities. The Journal of Knee Surgery, 28(2), 127–138.
3. Smith & Nephew. (n.d.) Journey II total knee replacement. https://www.smith-nephew.com/professional/products/all-products/journey-ii-cr/
4. DePuy Synthes. (n.d.). SIGMA total knee portfolio. https://www.depuysynthes.com/hcp/knee/innovations/sigma-high-performance-instruments
5. Stryker. (n.d.). Triathlon total knee system. https://www.stryker.com/us/en/joint-replacement/products/triathlon-total-knee-system.html
6. Zimmer Biomet. (n.d.). NexGen complete knee solution. https://www.zimmerbiomet.com/medical-professionals/knee/product/nexgen-complete-knee-solution.html
7. Hossain, F., Patel, S., Rhee, S. J., & Haddad, F. S. (2011). Midterm assessment of causes of failure of total knee replacement: comparison of computer navigation vs conventional total knee replacement. The Journal of arthroplasty, 26(3), 386–394.
8. Smith & Nephew. (2015). JOURNEY II BCS brochure. https://www.smith-nephew.com/global/assets/pdf/products/established-brands/journey-ii-bcs-usmc-brochure.pdf
9. Nicell, J., Shelton, C., & Tyber, J. (2019). Assessment of oxidized zirconium femoral components for total knee arthroplasty. Orthopedic Clinics, 50(2), 205-212.
10. Hallab, N., & Jacobs, J. J. (2017). Biologic effects of implant debris. The Bulletin of the NYU hospital for joint diseases, 75(2), 168–183.
11. Vertullo, C. J., Lewis, P. L., Lorimer, M., & Graves, S. E. (2019). The effect of implant weight on clinical outcomes of unicondylar knee arthroplasty: a blinded randomized controlled trial. The Journal of bone and joint surgery. American volume, 101(1), 42–49.
12. Kurtz, S.M., et al. (2002). Effects of radiation sterilization on polyethylene wear and tensile properties. Journal of Materials Science: Materials in Medicine, 13(11), 1077-1088.
13. Davis, N., Curry, A., Gambhir, A. K., Panigrahi, R., Walker, C., Thomas, M., & Simpson, A. H. (1999). Intraoperative findings in primary total knee arthroplasty. The Journal of bone and joint surgery. British volume, 81(4), 680–683.
14. Liu, A., Yue, E., Duan, S., Mani, S., Wang, P., & Meng, Y. (2015). Effect of femoral component design variables on wear of tibial insert in knee prosthesis using finite element analysis. Computational and mathematical methods in medicine, 2015, 823761.