Does lubrication of the TKR by synovial fluid need to be taken into account in estimating wear?

In Part 2 of this series (see here), I examined whether powerlifting would significantly reduce the lifetime of a total knee replacement (TKR).  I used Archard’s Law to do some analysis of the additional wear associated with the increased load on the joint of typical powerlifting training. Archard’s Law assumes a constant wear coefficient for the polyethylene bearing in the TKR so that the polyethylene wear rate is proportional to the load on the joint.

A reader asked me whether researchers have tried to model polyethylene wear rates, taking into account any lubrication effect that the body’s synovial fluids might provide to the prosthetic joints? If synovial fluids act as a true lubricant to prevent wear contact between the surfaces, then Archard’s Law may not apply.

How lubrication affects wear

Synovial fluids are complex fluids responsible for the lubrication present in our joints. These fluids consist of various constituents, including hyaluronic acid, surface-active proteins (i.e., lubricin), surface-active phospholipids, as well as various other proteins, each of them playing an essential role in lubrication.

Kung et al. (2015) reviewed 11 papers that examined synovial fluids providing lubrication to knee prostheses after joint replacement. They found that the cells in the fluid were similar to those in normal joints, protein and phospholipid concentrations were similar, but that the synovial fluids around joint replacement devices were typically lower in viscosity than pre-arthroplasty fluids. They concluded that the lubricant formed after joint replacement was adequate for good performance in most cases, but that studies were needed on its role in component wear or failure.

The primary purpose of lubrication is to create an acceptable lubricant film to sufficiently keep the two moving surfaces apart while allowing them to move with reduced friction. This is the ideal condition, but a lubricant can pass through several different regimes before it achieves this full film format. These regimes are associated with different frictional levels and can effectively result in non-linear changes in wear as a function of load (Mang and Dresel 2007, Maru and Tanaka 2007).

Lubrication works by separating surfaces with a film of fluid, but in boundary and mixed lubrication regimes, there are still contact points called asperities. When the load increases, these asperities come into greater contact, leading to increased wear and adhesion. 

The effective distance between the surfaces is the main parameter that influences lubricated wear behaviour. This distance usually is measured in terms of the ratio between the effective distance between the surfaces and the root mean square of roughness height (λ). In machines, the main factors that reduce λ are increased load or surface roughness and reduced relative speed or lubricant viscosity.

The following Figure illustrates the relation between the friction coefficient (μ), the wear coefficient (k), and λ. Values of λ greater than 3 results in little or no solid contact, which causes negligible wear and increased wear and adhesion. If changes in load result in transition across  any of these regimes, this can effectively result in non-linear changes in wear rate as a function of load.

Increasing the load on sliding surfaces should reduce the thickness of the lubricant layer and increase the wear coefficient. However, I came across one paper which carried out experiments with lubricated metal-metal contact and found that the wear coefficient effectively decreased as load increased so that wear was less than predicted by Archard Law’s proportionality to load (Ideris et al 2023).

Modelling the impact of synovial fluid lubrication on prosthesis wear rates

As for the modelling of the effect of synovial fluid lubrication in reducing wear in a TKR, researchers are only beginning to grapple with the challenges of determining the impact of synovial fluids on hip and knee prosthesis wear (Marian et al 2021, Gao et al 2022)

However, the analysis I performed in my previous article used observed failure rates of TKRs and such failure rates already include whatever impact synovial fluids have in lubricating the prostheses. Where the simplistic analysis I made using Archard’s Law may go wrong is in its assumption of a constant wear coefficient – meaning that that wear is proportional to load according to Archard’s Law. If changes in the lubricants film thickness or flow characteristics are associated with increased load, it possible that the additional wear associated with the additional load during powerlifting training may be greater than that estimated assuming its proportional to total load.

Potential impact of powerlifting on prosthesis failure rates

My previous calculations estimated that my typical training regime might result in a 1-2% increase in wear debris overall, and this would correspond to a 1-2% increase in the revision rate at 20 years from 8.7% to 8.9%.  Suppose that deviations from Archard’s Law associated with synovial lubrication resulted in the additional powerlifting load adding double or triple that percentage to wear debris. The revision rate at 20 years might rise from 8.7% to 9.0 or 9.2%.

We also need to factor in the continuing improvements in prosthesis failure rates. The failure rates I used in my previous calculations came from an analysis of data from national joint replacement registries and the 20-year failure rates would have mainly related to TKRs carried out in the late 1990s. My prostheses implanted in 2022 and 2025 almost certainly have lower average failure rates. 

The most recent data I could find from the Australian national registry had 20-year revision rates of 7.7%, 7.6% and 8% in 2023, 2024 and 2025 (Smith et al 2023, Lewis et al 2024, Lewis et al 2025). I think its reasonable to assume that improvements in failure rates for prostheses of the 2020s compared to those of the early 2000s have more than outweighed any increase in failure rate associated with powerlifting training.

Pending improved data on the relationship of prosthesis wear to load, I stick by my conclusions in the previous article. “Powerlifting training after joint replacements by an experienced lifter with attention to technique and careful progression appears unlikely to significantly decrease hip or knee replacement lifetimes. Indeed, the improvements in muscular strength around these joints from training may result in less forces acting in the joint across all activities and more than offset the effects of higher loads on wear.

My second total knee replacement and rehabilitation

I had my second TKR of the right knee just over four months ago on 2 September 2025. I saw a physiotherapist twice a week for three months and improved my maximum knee flexion to 140 degrees. My left knee, three and a half years after TKR, has a maximum flexion of 145 degrees. However, I had some pain in flexing my right knee through 90 degrees which took close to four months to fully go away.

Just a week ago, I did a walk in the French mountains to the east of Geneva with my son. It had snowed the night before and we walked 14 kilometres involving an 800-metre ascent and descent on sometimes steep and icy trails. I had no pain whatsoever in either knee and felt like I was definitely back to good functioning, even if needing a little more cardiovascular endurance. I include a  few photos below.

At the three-month mark, I started powerlifting training again. Initially with an empty bar, and then a slow and steady progression to build strength not only in the muscles but also in the tendons and ligaments. Currently I am deadlifting 100 kg for sets of five and squatting below parallel with 70 kg for sets of five. Today I tested what I could lift for 1 rep, achieving 90 kg for the squat and 130 kg for the deadlift. If my training continues to go well, I hope to compete in late March, aiming for squat around 100 kg or more and a deadlift around 160 kg or more.

REFERENCES

Gao L, Lu X, Zhang X, Meng Q, Jin Z (2022). Lubrication Modelling of Artificial Joint Replacements: Current Status and Future Challenges. Lubricants. 2022; 10(10):238. https://doi.org/10.3390/lubricants10100238

Ideris M, Kamaruddin S, Sulaiman M, Sukindar NA, Azhar A, Yasir A. (2023). Effects of Coating and Lubrication on Friction and Wear for Metal-to Metal Application. Journal of Advanced Research in Applied Mechanics. 110. 52-62. 10.37934/aram.110.1.5262.

Kung MS, Markantonis J, Nelson SD, Campbell P (2015). The Synovial Lining and Synovial Fluid Properties after Joint Arthroplasty. Lubricants. 2015; 3(2):394-412. https://doi.org/10.3390/lubricants3020394

Mang T, Wilfried Dresel W (2007). Lubricants and Lubrication. Second, Completely Revised and Extended Edition, Wiley and Co, 2007. https://onlinelibrary.wiley.com/doi/book/10.1002/9783527610341

Marian M, Shah R, Gashi B, Zhang S, Bhavnani K, Wartzack S, Rosenkranz A (2021), Exploring the lubrication mechanisms of synovial fluids for joint longevity – A perspective, Colloids and Surfaces B: Biointerfaces, Volume 206, 2021, 111926, https://doi.org/10.1016/j.colsurfb.2021.111926.

Maru MM, Tanaka, DK (2007). Consideration of stribeck diagram parameters in the investigation on wear and friction behavior in lubricated sliding. Journal of the Brazilian Society of Mechanical Sciences and Engineering, 29(1). https://doi.org/10.1590/s1678-58782007000100009

Lewis PL, Gill DR, McAuliffe MJ, McDougall C, Stoney JD, Vertullo CJ, Wall CJ, Corfield S, Du P, Holder C, Harries D, Edwards S, Xu A, Lorimer MF, Cashman K, Smith PN (2024). Hip, Knee and Shoulder Arthroplasty: 2024 Annual Report, Australian Orthopaedic Association National Joint Replacement Registry, AOA: Adelaide, South Australia. 2024. https://doi.org/10.25310/GLOL7776

Lewis PL, Gill DR, McAuliffe MJ, Stoney JD, Vertullo CJ, Wall CJ, Corfield S, Esaian R, Moylan S, Du P, Holder C, Edwards S, Xu Q, Oakey H, Lorimer MF, Smith PN (2025). Hip, Knee and Shoulder Arthroplasty: 2025 Annual Report, Australian Orthopaedic Association National Joint Replacement Registry, AOA: Adelaide, South Australia. 2025. https://doi.org/10.25310/MXFR3061

Smith PN, Gill DR, McAuliffe MJ, McDougall C, Stoney JD, Vertullo CJ, Wall CJ, Corfield S, Page R, Cuthbert AR, Du P, Harries D, Holder C, Lorimer MF, Cashman K, Lewis PL (2023). Hip, Knee and Shoulder Arthroplasty: 2023 Annual Report, Australian Orthopaedic Association National Joint Replacement Registry, AOA: Adelaide, South Australia. 2023. https://doi.org/10.25310/YWQZ9375