The number of people with lower limb amputation in the United States is projected to approach 4 million individuals by 2050, with over half being transtibial amputations (TTA) (Ziegler-Graham et al., 2008). Unfortunately, people with limb loss experience reduced mobility and quality of life compared to able-bodied individuals (Gailey et al., 2008). Prosthesis socket-related pathologies and secondary comorbidities (e.g., low back pain (LBP) or osteoarthritis (OA)) are highly prevalent in this population, resulting in reduced mobility, quality of life, prosthesis abandonment, and increased mortality rates (Gailey et al., 2008, Kulkarni et al., 1998, Reiber et al., 2010, Sinha et al., 2011, Sprunger et al., 2012).

Compensatory gait patterns required for the loss of ankle plantarflexor muscles are well documented in individuals with unilateral TTA using a socket prosthesis, which is primarily attributed to a change in force transmission between the ground and limb that will influence kinetics required for ambulation. For example, patients with TTA commonly increase amputated limb hip extensor power during pre-swing phase to aid in forward propulsion (Nolan and Lees, 2000, Winter and Sienko, 1988) and demonstrate increased motion of the trunk in the frontal and transverse planes to aid in dynamic stability (Hendershot and Wolf, 2014, Molina-Rueda et al., 2014, Rueda et al., 2013). Although necessary, these compensations commonly result in asymmetric limb loading, most commonly underloading the residual limb and overloading the intact limb (Mattes et al., 2000, Molina-Rueda et al., 2014, Nolan and Lees, 2000, Sanderson and Martin, 1997). Over time, habitually asymmetric joint loading is known to increase the risk of additional secondary comorbidities (Ehde et al., 2001, Gailey et al., 2008, Kulkarni et al., 2005, Morgenroth et al., 2012, Nolan et al., 2003, Norvell et al., 2005). As a result, LBP has been reported in upwards of two-thirds of this population (Kulkarni et al., 2005) with individuals 1.4 times more likely to experience OA (Struyf et al., 2009), which can have a substantially negative effect on mobility and quality of life (Gailey et al., 2008, Hungerford and Cockin, 1975, Kulkarni et al., 2005).

Bone-anchored limbs that directly attach the prosthetic limb to the residual bone via an implant within the intramedullary canal of the residual limb are an alternative to socket prostheses (Brånemark et al., 2001). Socket-suspended prostheses introduce socket-related pathologies from both direct contact with soft tissues (i.e. skin rubbing and slippage between the skin and socket) (Dickinson et al., 2017, Mak et al., 2001, Sanders et al., 1992) and residual limb motion within the socket (Baumann et al., 2022, Breen and Dupac, 2016, Commean et al., 1997, Maikos et al., 2021, Noll et al., 2015) that may contribute to compensatory mechanisms developed as a means to increase stability and reduce areas of pressure and discomfort (Lloyd et al., 2010, Mak et al., 2001, Seyedali et al., 2012). The direct fixation of the prosthetic limb to the residual bone eliminates socket-related pathologies and provides a more normative load transmission through the residual limb (Robinson et al., 2020), thus potentially altering compensatory movement patterns adopted with socket prostheses.

As lower-extremity bone-anchored limbs originated for the transfemoral amputation population, most of the research is focused on that population with little known on how this prosthesis impacts the TTA population. Preliminary outcomes from individuals using bone-anchored transfemoral prostheses have indicated an increase in physical function through increased mobility, physical activity, time of prosthesis wear, balance confidence, and independence from assistive devices (Al Muderis et al., 2016, Atallah et al., 2017, Davis-Wilson et al., 2022, Gaffney et al., 2023, Gaffney et al., 2022a, Leijendekkers et al., 2019, Sullivan et al., 2003). During rehabilitation, emphasis is placed on minimizing aberrant trunk and pelvic motions to restore symmetry to an inherently asymmetric system after lower limb amputation (Leijendekkers et al., 2017b). Individuals with transfemoral amputation using bone-anchored limbs have also demonstrated improved biomechanical symmetry (Darter et al., 2023, Davis-Wilson et al., 2023, Tranberg et al., 2011, Vandenberg et al., 2023). Notably, prior evidence has shown that patients exhibit decreased lumbopelvic ranges of motion and coordination (Darter et al., 2023, Gaffney et al., 2024, Tranberg et al., 2011), increased amputated limb muscle force production (Gaffney et al., 2022b, Vandenberg et al., 2023), and improved hip joint loading symmetry (Davis-Wilson et al., 2023, Vandenberg et al., 2023) when using a bone-anchored limb as compared to a socket prostheses. Collectively, these biomechanical changes in symmetry in patients with transfemoral bone-anchored limbs may have a positive impact on the etiology of secondary comorbidities. However, the impact of transtibial bone-anchored limbs on joint loading remains unknown, and thus its impact on secondary comorbidities is not well understood.

The objective of this study is to determine movement pattern and joint loading changes in participants with unilateral TTA before and 12-months after bone-anchored limb implantation using musculoskeletal modeling. We hypothesized that trunk range of motion will decrease, resulting in improved bilateral hip and knee joint loading symmetry, 12-months after bone-anchored limb implantation.