Golf is generally considered a low-impact sport. But if the knee loads generated during a golf swing were enough to take down Tiger Woods, what are the implications for athletes who have undergone total knee arthroplasty?

by Judy A. Blake, BA, Nikolai Steklov, BA, Shantanu Patil, MD, Clifford W. Colwell Jr., MD, and Darryl D. D’Lima, MD, PhD

The emphasis on exercise, fitness, and wellness has become an integral part of the life style for the Baby Boomer generation. The commitment to fitness has had long-term effects because of the stresses and strains on the joints, which has resulted in younger, more active patients suffering from joint diseases and requiring implants earlier in life. Arthritis is one result from the breakdown of cartilage, which ultimately requires joint replacement for many. This breakdown of cartilage, most commonly from osteoarthritis, is the leading cause of disability in the United States, with total costs estimated at $128 billion per year.¹

The aging Baby Boomers will start turning 65 years old in 2011. According to a study by Kurtz et al, this will increase the demand for joint replacement surgery by 673%, to 3.48 million procedures performed annually by 2030.² The projections also indicate that the demands for re-operations for complications will double by 2026.³ Allografting to repair articular cartilage has been shown to be successful in younger patients, but does not preclude the possibility that joint replacement may be required in the future.^4-7 This is an important consideration since patients who have led active lifestyles typically plan to continue their same activity levels after receiving implants.

The implications of knee injury have gained increasing prominence, with Tiger Woods making front page news not just for winning the 2008 U.S. Open golf tournament in San Diego, but for doing so on a left knee ravaged by a ruptured anterior cruciate ligament, a proximal tibial plateau fracture, and associated cartilage damage. His win in this history-making tournament brought knee-injury concerns into the spotlight as never before.

Previously, forces exerted on the knee during golfing were thought to be minimal. However, recent research from the Scripps Clinic⁸ confirmed that that kind of force was enough for Woods’ torn anterior cruciate ligament to allow his femur to slide anteriorly to the tibia, resulting in a compression fracture of the tibial plateau.⁹

An understanding of the biomechanics of the knee joint are crucial for analyzing the effect of the forces on the joints during activities of daily living that primarily involve the lower extremities. Abnormal knee joint loading is associated with cartilage degeneration and osteoarthritis (OA).¹⁰ A study of more than 3000 subjects conducted by Murphy et al found that the lifetime risk of knee OA rose with increasing body mass index (BMI), with a risk of 66% among those who were obese.¹¹

When total joint replacement was first developed, it was intended to primarily treat an elderly population.² But with younger and more active patients presenting with joint disease that requires implants, device companies have had to improve the materials and  implant design.¹² By way of example, a study by Bozic et al analyzed alternate materials for the bearings in hip arthroplasty for cost-savings and cost-effectiveness based on willingness to pay. The study evaluated materials such as highly cross-linked polyethylene, more costly metal-on-metal and ceramic-on-ceramic, and even more costly alternatives being explored such as diamond-on-diamond.  The increased costs varied from an additional $500 to $4000 per procedure. The authors concluded that the increased cost is dependent on the patient’s age at the time of surgery and the reduction in the probability that the patient will require a revision.¹³

The lengthening life expectancy and higher overall fitness of the age group of patients undergoing knee arthroplasty continue to raise the bar on durability and survival of the prosthetic components. It is not known which activities generate forces within a range that is physiological desirable and do not jeopardize the durability of the arthroplasty. The conventional wisdom is that low-impact activities (such as biking, walking, and golf) are safer than high-impact activities (such as jogging and tennis).  Practitioners’ recommendations are based on subjective opinions without quantitative measurements of knee forces, and are not always consistent.^14,15 Clinical outcomes are also mixed, with some studies reporting increased component failure in more active patients, while others reporting no difference.^15-20

Historically, predictions of the forces on the knee were based on theoretical data that estimated the forces ranging from three to six times body weight for walking, ascending and descending stairs, and isokinetic exercises.^21-23 Even though this information has been useful for implant design, the ability to directly measure the actual forces would have a significant influence on not only implant designs, but also would provide physicians with information that could be used to improve postoperative exercise programs and orthotics for patients.²⁴

To accomplish this direct measurement of joint forces, our goal was to develop an electronic tibial component of a total knee prosthesis, instrumented with multichannel transducers (load cells), a microtransmitter, and an antenna. After years of developing the design, and through several design changes, a prototype finally passed the numerous fatigue tests in our knee-wear simulator.²⁵ The prototype was tested in cadaver experiments, followed by intraoperative testing.^{22, 26}

After appropriate institutional review board approval, the electronic knee prosthesis was implanted in a patient in February 2004. The first-generation implant, a custom tibial prosthesis based on the PFC Sigma design, was manufactured by DePuy Orthopaedics and had four load cells to measure the forces.²⁶ A second-generation design, which was subsequently implanted in three patients, was manufactured by Zimmer, based on the Natural Knee II (NK-II) tibial tray design that measured all six components of force including shear and torque.⁸ The patients who received the electronic components range in age from 67 to 83 years. Two patients had a left TKA and one had a right TKA. All lead very active lifestyles. They play golf and tennis; one patient skis on a regular basis. All the patients played golf right-handed, but one was ambidextrous and could hit the ball from either side.

Peak axial knee forces were measured in all four patients (average of about six trials per activity per subject), and tibial shear and moments were measured in the three patients implanted with the second-generation implant.  In-depth biomechanical analyses were performed for each activity relating tibial forces to knee kinematics and correlating with ground reaction forces, external flexion and adduction moments.

Recreation and exercise

To better understand activity-specific forces on the knee joint, we directly measured tibial forces in vivo during exercise and various recreational activities at one year following the implantation of the instrumented prosthesis.⁸ The patients who have received the electronic implant were analyzed at the Shiley Center for Orthopaedic Research & Education at Scripps Clinic, at the TaylorMade Performance Lab, Carlsbad, CA, and on a driving range (Torrey Pines Golf Course, La Jolla, CA).

The compressive force xBW were measured using a driver, a 7 iron, and a sand wedge. The speed was measured from 0 to 3 seconds (time to impact). The forces generated during simulated swinging in the laboratory (without a ball) were similar to those generated on the driving range. High tibial forces were generated during the golf swing at impact. Much higher forces were generated in the leading knee (left knee in a right-handed golfer) relative to the trailing knee. In addition, a golf swing with a driver tended to generate higher forces than with a sand wedge. The peak axial forces at impact ranged from 1.5 to 3.5 times bodyweight (xBW), with the club speed ranging from 30 to 75 mph (Figure 1).  The forces for each club were similar, with the peak forces of 4.5 xBW measured at the 1-second point of impact. The forces gradually decreased from 4.5 xBW to 1-1.5 xBW at the 3-second point of impact. The driver generated the highest peak forces, although there was no statistically significant difference between clubs, given the small sample size. Anterior tibial shear (0.34 ± 0.01 xBW) and axial tibial torque (13.0 ± 0.33 N-m) were in the moderate range.

We compared the data collected during the golf swing against a wide variety of activities that we had previously reported,⁸ including tennis, which are of interest to both younger and active older populations.

Tennis:   Tennis was analyzed in the laboratory as well on the tennis courts (La Jolla Beach and Tennis Club, La Jolla, CA) during actual play. Mean peak forces generated during the serve and during a forehand return were higher than those generated during the backhand return. (Figure 2 compares the knee forces for all activities.) Anterior shear was moderate (0.28 ± 0.12 xBW). Forces measured during actual play on the tennis courts were on average 12% higher than those generated during simulated strokes in the laboratory.

Walking: Peak tibial forces during walking increased steadily over the first 12-month postoperative period and remained steady after that at a mean of 2.5 (±0.4) xBW.   Treadmill walking (2.05 ± 0.20 xBW) was lower than those measured during level walking on a laboratory floor (2.6 xBW).  Treadmill speed during comfortable walking (range, 1–3 miles/hour) had no effect on peak tibial forces.  Power walking (4 miles/hour) generated significantly higher forces on the treadmill (2.80 ± 0.43 xBW).

Jogging:  Jogging trials took place in the laboratory on a hard floor. Peak forces recorded during jogging were even higher than those recorded during power walking. The peak ground reaction forces, however, only increased by a mean of 40% when jogging relative to walking on the laboratory floor, which indicated that muscle forces contributed substantially to the increased knee forces.

Stair Climbing:²⁷ Peak tibial forces during stair climbing increased to a mean of 3.2 xBW at two years. This result was substantially lower than that calculated by computational models.²⁸

Chair rise: Peak tibial forces were not affected by changing seat height. Peak anterior shear components were small (range, 0.1–0.3 xBW) but varied with seat height.²⁹

Bicycling: Stationary bicycling was analyzed at various levels of difficulty and speeds ranging from 60 to 90 revolutions per minute (rpm). Overall, tibial forces peaked at 1.03 ± 0.20 xBW.  Increasing the speed of bicycling from 60 to 90 rpm did not affect peak tibial forces, but did affect the flexion angle at which the tibial force peaked.⁸

Rowing, Elliptical Trainer, and StairMaster:  Rowing (Indoor Rower, Concept II, Morrisville, VT), generated a peak mean tibial force of 0.85 ± 0.08 xBW at a maximum flexion angle of 90 ± 1°.  Exercising on the Elliptical Trainer (9500 HR, Life Fitness, Schiller Park, IL) generated a mean peak tibial force of 2.24 ± 0.22 xBW, and increasing levels of difficulty showed no change. Exercising on a stair-climbing machine (StairMaster 4000 PT, Nautilus Inc, Vancouver, WA) generated similar forces of 2.5 xBW⁸ at lower levels of intensity but increased to more than 3 xBW at higher levels. The knee flexion angle at peak tibial force also increased with increasing intensity.

Leg Press and Knee Extension Machines:    Peak tibial forces increased with increasing resistance during the leg-press activity, but not during the knee extension activity. Both the leg-press and the knee-extension activities generated similar magnitudes of peak anteroposterior shear (0.24–0.35 xBW).  The results for all activities are shown in Figure 2.

Summary

The results of our series of related studies support a scientific approach to recommending activities after TKA. Tibial forces comprise only some of the factors that may contribute to the potential for prosthetic wear and damage. Other factors include joint kinematics, which affect contact area, and the number of cycles of the activity. For example, the golf swing generated forces similar in magnitude to jogging. However, the number of cycles during which the knee is exposed to high forces during a golf game are fewer than the number of cycles during jogging. Activity-specific knee implants can now be designed, such as a device that might be made more tolerant of golfing by optimizing the conflicting needs of increased rotational laxity and conformity.

Golf is a popular sport that typically places low physical demands on the body and one of the few sports played by the older, retired population. Mallon et al in their 1993 report used the Knee Society questionnaires to assess pain experienced by golfers who had undergone TKA, and concluded that the greatest torque occurred when the golf club made impact with the ball.³⁰ A report by Bradbury came to the same conclusions with regard to the downswing.³¹ Gatt et al in a 1998 study reached the same conclusion that peak forces were reached during the downswing.³² It was hypothesized that a more skilled golfer might have a more powerful swing, which would lead to greater forces. On the other hand, professional golfers might have a more efficient swing, which would reduce loads on the knee.

The subjects implanted with the electronic knee prostheses generated up to 4.5 xBW at a club head speed of 70 mph. Extrapolating these forces to those generated at Tiger Wood’s club head speed would result in forces close to 9x BW and would explain the reason for his knee injury.³³ Sports magazines have reportedWoods’ golf swing speed to be around 125–140 mph. Most PGA pros hit at around 100–110 mph, although other golfers like Bubba Watson have recorded swing speeds at 150 mph in practice sessions. Meyers et al in a cadaveric study estimates 5.4 ± 2 kN as the average force at which the ACL ruptures in a cadaveric knee, which is around 5200N. Tiger Woods weighs approximately 180 lbs, which is around 800 N. Nine times that would be 7200 N or 7.2kN, which is just around the upper limit of the average forces.^34,35

Mallon’s study concluded that although members of the Knee Society do not discourage their TKA patients from playing golf, those patients may experience pain. Patients are advised by their surgeons to wait 18 weeks before beginning play. Two thirds of the members recommend using a golf cart while playing.³⁰ A survey of the American Association of Hip and Knee Surgeons (AAHKS) published by Swanson et al listed recommendations of side-by-side activities of daily living and sports. Golf was in the “unlimited” range in a comparison of activities, which were categorized as discouraged, occasional, or unlimited.³⁶ This survey also referenced our study with regard to the high force generated on the leading leg during the golf swing.⁸

Bradbury et al³¹ concluded that sports activities have a protective effect on the bone-implant interface by encouraging bone regrowth. The study concluded that a stress threshold may be important, but to date this has not been identified. In general, the benefits of low-impact activities have been well-established.

A better understanding of the tibiofemoral forces in vivo will help improve implant design and refine the theoretical need models. This will ultimately help the growing number of patients who suffer from osteoarthritis of the knee.³⁷ In addition, with younger patients requiring implants, who along with the Baby Boomer generation expect to continue their same exercise regimen,³¹ understanding the knee forces is crucial for the future of implant designs and how confident practitioners can be in recommending activities for their patients.

Judy Blake, BA, is a medical writer and copyeditor at the Shiley Center for Orthopaedic Research and Education at the Scripps Clinic in San Diego. Nikolai Steklov, BA, is a biomechanical engineer and Shantanu Patil, MD, is an orthopaedic surgeon and research scientist at the same institution. Clifford Colwell Jr., MD is director of the Shiley Center for Orthopaedic Research and Education and clinical professor in the department of orthopaedic surgery and rehabilitation at the University of California, San Diego, School of Medicine. Darryl D’Lima, MD, PhD, is director of orthopaedic research laboratories at the Shiley Center for Orthopaedic Research and Education and assistant professor and researcher in the division of arthritis research at The Scripps Research Institute.
Financial disclosure: Zimmer funded a research project to collect some of the data discussed in this article.

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Figure Legends

Figure 1: Representative graphs of knee forces generated during a golf swing when using a driver, a 7 iron, and a sand wedge.

Figure 2: Comparisons for mean peak forces for various activities. The red star indicates higher than expected forces and green indicates lower than expected forces.