Physical, Chemical, and Biological Compatibility of Hip Prosthetics
Have you ever changed a flat tire? It’s often as straightforward as replacing the flat with a new spare. You can fix pretty much all simple mechanical and biomedical issues like this. If you need an organ, you find a donor match. If you need an immunity, you get a vaccination. But how would you heal an injury when an injection, transplant or self-graft is not possible? The development of prosthetics within the orthopedic field centers around these issues. Doctors created the earliest prosthetics around the eighteenth century but quickly discovered the many physical, chemical, and biological constraints. They tested a wide variety prosthetics in trial-and-error to find the perfect material. Doctors tried gold, wood, and even pig bladder to address load bearing, chemical corrosion, and biological compatibility within the human body (Merola and Affatato). Although today we implement more standardized materials, this orthopedic field proves fascinating for its convoluted solution to such a superficially simple problem. It’s amazing that we can approach the same problem from so the depths of such different disciplines. In this article, we will be discussing prosthetics’ physical, chemical, and biological limitations, and revel at how beautifully the sciences integrate to ultimately transform common inorganic materials into our own flesh and blood.
Although there’s many types of prosthetics, we will be focusing on total hip replacements because of how common the procedure is. Our hip joints are composed of two major parts: the round femoral head and the hip socket (fig. 1). When healthy, the round surface of the femoral head smoothly rotates against the pelvis. In addition to this ball-and-socket like mechanism, muscles spanning our abdomens and knees gives our hips an incredible range of motion. However, a chronic degenerative disease, acute fracture, or other injury may warrant an entire replacement of the femoral head. The specific material or combination of materials your doctor selects must maintain your fluid range of motion, support your body weight, resist falling apart within you, and not trigger your immune system to attack the prosthetic.
Imagine doing jumping jacks. Your hips adduct and abduct thighs in and out along a single Cartesian plane (fig. 1). Now imagine doing jumping jacks for an hour. Your muscles are
pretty tired, and you’re probably regretting spending quarantine on the couch. The soreness in your muscles is indicative of them exerting a muscular force Fm (N) to keep doing jumping jacks. When you finally collapse of exhaustion, your hip joint may feel tired or heavy from supporting your weight. The weight of your torso on your hips is a reduced gravitational force called Fg’ (N) since it is not your full body weight. For you to do jumping jacks in the first place, the sum of the muscular and gravitational forces must equal some non-zero constant. By Newton’s second law, this non-zero net force causes acceleration of the joint so that:
ΣF = Fg’ + Fm = ma.
Of course, if you’re aware of the most general expression of Newton’s second law, we assume you’re not losing significant weight from one day of exercise. Due to the complementary nature of muscle groups pulling in opposite directions, the net force may point in the positive or negative directions. In addition to withstanding the forces from your movement and weight, prosthetics must also share similar physical properties with the bone they are attached to. Elastic deformations or how much the prosthetic ‘gives’ as well as load bearing strength are two important examples. If a prosthetic is too elastic or stiff, the bone will experience stress shielding where the bone will adapt to bearing a decreased loading and will weaken as a result. The weakened bone will cause the prosthetic to loosen over time and will need to be replaced (Kunčická, Kocich and Lowe) (Aherwar, Singh and Patnaik).
Overall, the hip joint is responsible for moving and load bearing across multiple coordinate systems for extended periods of time and repetitions while maintaining physical consistencies with a neighboring boney support (Kunčická, Kocich and Lowe) (Eschweiler, Kabir and Gravius). If the prosthetic cannot support the net force generated by your muscles, it may shatter. Such was the case with an experimental glass femoral head created by an American surgeon in the early twentieth century. If the prosthetic does not share the physical properties of the neighboring bone, it may loosen, cause the neighboring bone to deteriorate, and warrant another surgery. Finally, if the prosthetic rubs against the hip socket with too much friction, it may release foreign particles into the body. Not only are these particles potentially harmful by themselves, but they may also trigger an infection. The buildup of inflammatory cells between the bone and prosthetic will cause loosening. From a purely physical perspective, current metal-on-metal prosthetics excel for their less brittle composition that allows for larger diameter femoral heads. This larger diameter prevents future dislocations of the hip joint (Knight, Aujla and Biswas). Metal-on-metal prosthetics also have high wear resistance and low frictional forces ensuring higher durability.
From a chemical perspective, prosthetic materials must withstand the corrosive environment of the human body. Most metals will readily lose their electrons when placed in the body’s oxygen-rich environment. In addition to frictional forces, chemical reactions may leak potentially dangerous metal ions into the surrounding tissue. Although doctors use inert, biocompatible coatings, prosthetics may still crack or stress fracture under consistently high mechanical loads (Kunčická, Kocich and Lowe). The metal-on-metal prosthetics fail in this regard, as doctors are now becoming increasingly concerned with the chronic effects of liberated metal ions. Although scientists apply anti-oxidative protective coatings, long term wear can break down this layer. (Merola and Affatato). As a result, scientists developed ceramic-on-ceramic prosthetics that release few, inert particles and can also withstand daily activity. However, the implementation of ceramic-on-ceramic prosthetics remains expensive because of the specialized nature of the surgery and the materials themselves (Knight, Aujla and Biswas).
Finally, prosthetic materials must not trigger an allergy, infection, or any other biological response to maintain biocompatibility. As previously stressed, an autoimmune response may lead to an infection and loosening. This poses a challenge with prosthetics containing iron, nickel, and magnesium: these metals are prone to corrosion, and their ions are toxic to living cells (Kunčická, Kocich and Lowe). Unfortunately, metal-polyethylene prosthetics today suffer similar issues. The released plastic polyethylene particles trigger an infection and cause loosening. Despite this awful consequence, they remain popular due to their overall safety, predictability, and cost efficiency (Knight, Aujla and Biswas). Altogether, very few if not no substance is completely biocompatible and corrosion resistant. The different properties of different materials forces doctors to either combine materials or rely upon biocompatible surface coatings (Kunčická, Kocich and Lowe).
Clearly, no prosthetic material is perfect. We’ve made significant improvements since the days of glass and pig skins, but the individual nature of each prosthetic makes hip replacements much more personal than a vaccination or organ transplant. Each material exists on a spectrum of unique physical, chemical, and biological factors. In the end, doctors must decide which combination of materials works best for each of us. Depending on our ages, activity levels, and financial situations, we may receive very different prosthetics that will determine how we carry out the rest of our lives. Regardless, hip prosthetics epitomize the frontier of materials science
engineering’s capability to improve our quality of life and extend our legacies beyond their natural limits.
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Allison is a Materials Science Engineering senior at Emory University. She researches single-molecule biophysics in the Finzi-Dunlap group and is passionate about applying physics to biological systems. In her free time, she plays on the club volleyball team, tries new Kombuchas, and watches hockey.
