Question:

Critically review the literature and to apply both basic engineering mechanics and the information that they learn about joint replacement function within lectures to a clinical biomechanics problem.

Answer:

Abstract
The involvement of Biomedical Engineers in arthritic joint replacement includes, but is not limited to designing improved surgical methods using biomechanical concepts and use of suitable biomaterials for joint implants. Surgical methods used in toe arthritis have been immensely improved by biomedical engineering concepts. They include application of biomechanical concepts in external fixators, total joint replacement designs, improvement of joint replacement implants using biocompatible materials and finally the development of digital arthrodesis. Despite the advancements in this regard, there is still need for further improvement of the efficacy and safety of these techniques, thereby influence the long-term outcome.

Introduction
Deformities at the toes and toe joints are frequent consequences of Osteoarthritis and Rheumatoid arthritis. Hence, ‘toe arthritis’ represent a significant clinical delinquent that has been persistent for a long time. Biomedical engineers working in concert with the doctors have led to clinically relevant developments to treat toe arthritis, especially in case of replacement of toe joints. Two of the most commonly seen toe arthritis conditions are Hallux rigidus (HR) and hammer toe, which are generally associated with Rheumatoid arthritis as well as Osteoarthritis (Coughlin et al., 2013). For instance, the prevalence of HR in adults older than 50 years old is ~2.5% (Hamilton et al., 1997).

Toe arthritis conditions are usually characterized by restricted motion, as the mechanical properties of the first metatarsophalangeal joint (MPJ) or lesser toes are severely compromised. In the case of MPJ, when the dorsiflexion is reduced from 40-60º to less than 10º, a treatment approach is required to correct the deformity or reconstruct the toe (Webb et al., 2005). Therefore, many of the surgical treatment strategies were implemented to improve the motion of the joint while restoring the normal foot functions. Biomedical engineering has incorporated biomechanical applications and established biocompatible materials to be used in toe replacement strategies. This mini-review will provide how ‘engineering’ concepts are being applied to address a clinical problem, in this case the ‘toe arthritis’. Two biomedical engineering aspects are being reviewed here. They include biomechanical engineering aspects and biomaterial biocompatibility aspects.

Literature Review
Non-surgical and surgical techniques for toe arthritis treatment: The non-surgical therapy strategies applied in this situation include pain relievers, anti-inflammatory drugs, heat-cold therapy, nonsteroid/ cortisone injections to the joint, shoe gear modifications, Morton's extension modified orthotics and carbon fiber inserts. These conservative techniques provide unsatisfactory outcomes in cases of advanced stages of arthritis.

The biomedical engineering aspects are mainly utilized in basic and improved surgical component of the therapy strategies. The most basic surgical methods include but are not limited to cheilectomy, fusion/arthrodesis, interpositional arthroplasty, decompression osteotomies and arthrodesis. The surgical toe replacement options are heavily dependent on the stage of degeneration of the joint. Bioengineering applications used in surgical options to treat progressing levels of HR can be taken for example. Cheilectomy and periarticular decompressive osteomy is used at early stage HR. In mid-stage HR complications, joint sparing techniques such as interpositional grafting and athrodiastasis as well as functional motion-sparing techniques such as total joint replacement are used. The end-stage HR are more degenerative arthritic stages which are difficult to be treated in simple techniques, but total joint replacement along with other joint-destructive procedures are used to alleviate the pain and/or motion difficulties (Perler et al., 2013).The following sections will discuss examples of Biomedical Engineering applications in toe arthritis.Your Name Module: Functional Joint Replacement Technologies

Usage of external fixator: In some of the toe arthritis treatment methods, an external fixator is used for correction of the deformities and reconstruction of toe/foot (DeHeer, 2003). Many of the toe treatments involve unilateral external fixator stability approach which is illustrated in Fig.1A. They also provide compression during the post-operative period. In Arthrodiastasis, a monorail mini-external fixator is used to move articular surfaces while facilitating stretch-relax mechanisms of the soft tissue at the MPJ (Fig.1B) (Perler et al., 2013). Usually the mini-external fixator creates gradual distraction of the joint space 0.5mm per day until it reaches a space 2-3 times bigger than the normal joint space. Moreover, interphalangeal arthroscopy is used as a corrective procedure for deformities in interphalangeal (IP) joint of the toes (Fig.1B) (Lui and Yuen, 2015). For example, in a patient with Hallux valgus and hypermobility, Tarsometatarsal joints (TMTJ) Arthroscopy was used on IP joint of the toes. The 1.9-mm 30 arthroscope provided extension and flexing the joint required for visualization. As an indication of the efficiency of the technique, analysis showed that hallux valgus angle and intermetatarsal angle were improved by 25.6° and 10.6°, respectively (Michels et al., 2011). Collectively, these examples demonstrate the use of external mechanical aspects in toe arthritis is beneficial in achieving functional toe motions.

Total Joint Replacement Designs: Another biomedical engineering advancement was the development of total joint replacement which was initially designed for fingers (Weems and Van Vo, 2013, Weems and Van Vo, 2014). However, as the mechanical properties of the figures and toes are fairly comparable, these biomedical calculations can be used in toe joint replacement as well (Weems and Van Vo, 2014). These are basically ‘single-piece prosthetic designs’ or implants used for metacarpophalangeal/phalangeal-phalangeal total joint replacements (Fig.2A) with varying degrees of peak stress under 50N load at the flexion centers (Example: Sutter design - 5.20 GPa; Swanson design - 8.71 GPa; Neuflex - 0.188 GPa) (Fig.2B-C) (Weems and Van Vo, 2014).

Biomaterials in joint replacement implants: The implant material is a critical factor in better performance of the implant in vivo, which adapt to the Wolf law of bone remodelling (Bronzino, 2006). The same biomedical group designed one-piece silicone implant device with many similarities to the natural finger/toe motion mechanics (Fig.3A) (Weems and Van Vo, 2013). Moreover, the total finger/toe joint replacement assembly (Fig.3B) was simulated to adapt similar static and dynamic properties of the natural joint using the equations shown in Fig.3C. The silicone implant material used and the biomechanical design comparative to nature toe/finger allowed 3 and 4 MPa of stress tolerance at 10N load, which is equivalent to the maximum stress tolerated by a natural joint. Therefore, theseobservations highlight the suitability of silicone as a viable biomaterial when employed in concert with biocompatible mechanical simulations. Applying this concept, currently, there are commercially available toe replacement implants such as Integra® Silicone MTP Toe Replacement System where different geometries of implants are utilized based on the affected toe (Fig.3D).

However, due to reduced biocompatibility and longevity, the usage of silicone implants in joint replacement techniques have been less successful. For this reason, recent studies have been biased towards bipolar non-constrained Titanium, Polyetheretherketone (PEEK) or Silastic as the biomaterial for arthroplasty of the first metatarsophalangeal joints (Hetherington et al., 1994, Morgan et al., 2012). Computational calculations in (Weems and Van Vo, 2013) suggested solution treated titanium (Ti) alloy (6Ti-4Al-W) or Ultra high molecular weight polyethylene could be acceptable biomaterials to be used in single-piece joint replacement implants. The equation used in building theoretical three-dimensional model of the implant with different materials is as follows.