Where Are We Now?
While THA is considered one of the more successful orthopaedic surgical procedures, patients who have undergone THA retain abnormal gait patterns after surgery [3, 6]. Altered joint biomechanics arising from those gait patterns could place surrounding joints and perhaps even the surgical implant at risk [1, 4]. As the average age of THA patients decreases and their overall activity level increases, understanding postoperative changes in joint mechanics will become increasingly important. Patients who undergo THA generally see improvements in kinematic and kinetic parameters such as sagittal-plane ROM and peak hip adduction moment compared with their preoperative measures, but continue to see deficits in variables such as walking speed and frontal plane ROM compared with healthy controls [3, 6]. Additionally, abnormal loading of adjacent joints (including the ipsilateral knee and the lumbar spine, and even the contralateral hip and knee) may persist following THA, which may contribute to an increased rate of joint degradation at these surrounding joints [2, 5].
Therefore, it is important for orthopaedic surgeons to understand how joint biomechanics are altered in both the short and long-term following THA. But the majority of post-THA gait biomechanics studies have focused on kinematics without consideration for the loading that is involved, according to Bahl and colleagues . In fact, out of the 74 studies that met their inclusion criteria, only one kinetic variable, hip adduction moment, was examined and reported by more than two studies .
In the current study, Queen and colleagues  focused on identifying pre- and postoperative changes in sagittal-plane lower-limb power during walking and stair climbing in active THA patients. This study identified improvement in hip power and a reduction in compensatory power production from the surrounding joints following THA in relatively young, active patients . This paper emphasized the need to better understand the loading in all the lower limb joints, not just the operative joint, and to analyze additional activities of daily living other than just walking in order to better understand how movement and loading might influence the overall lower-limb joint health in THA patients. This may help surgeons make better implant choices (to match patients’ biomechanics), modify surgical approaches, and adjust rehabilitation protocols to individual patients.
Where Do We Need To Go?
As with anything in an evolving orthopaedic field, we need more high-quality data. Although plenty of studies have focused on how movement and loading are altered after THA [1, 3, 4, 6], these papers generally have a small sample size and would be underpowered to subgroup their tested sample by patient-specific parameters, such as activity level and post-surgery expectations, that might impact physical post-operative function and perceived surgical outcome. Future studies should build on the work of the current study by determining the potentially important variables, including surgical approach, implant type, rehabilitation protocol, and perhaps even patient activity level or post-operative goals. Some studies already have suggested that implant type  and surgical approach  may matter, and I suspect the other factors I mentioned may, as well. Queen and colleagues  attempted to compare the joint power differences between men and women following surgery since women have been reported to experience more limitations up to 5 years following surgery , but their statistical power to make that comparison is limited by their overall small sample size (13 men and 13 women). Larger studies are needed to move us towards a patient-specific approach to THA.
Additionally, there is a need to determine which biomechanical variables should be analyzed and how these variables are defined, measured, and ultimately standardized. Based on the review article by Bahl and colleagues , out of 74 studies that were included in their analysis, a total of 20 spatiotemporal, 56 kinematic, and 54 kinetic unique variables were identified. The large number of reported variables makes it difficult to identify what measurements are actually important as it pertains to patient outcome and function. Similarly, many of these parameters have more than one way in which they can be defined, making it difficult to compare the findings of one study and in one cohort of patients with those in another.
How Do We Get There?
Through discussions moderated by professional societies perhaps including the American Academy of Orthopaedic Surgeons or the Orthopaedic Research Society, researchers should come to an agreement about the biomechanical variables to be investigated, the best definitions for those variables, and the best methods to be used to investigate those variables. Additionally, specific funding should be designated from organizations such as the Orthopaedic Research and Education Foundation or through federal sources such as the NIH that is specifically aimed at multisite studies where the first aim is syncing the data collection and analysis methods so that large datasets can be generated. A larger and more-diverse dataset would allow for subgroup analyses to investigate the influence of particular patient variables.
Additionally, this large and diverse set of data could be used to help develop and validate more-predictive finite element and musculoskeletal hip models. Modeling is an efficient way to investigate the possible link between patient-specific variations and joint function. However, these models require extensive validation before they are useful. Eventually, long-term longitudinal studies that follow people prior to the development of osteoarthritis (OA) should be developed. One glaring weakness that still exists in every movement-related THA study to date is the lack of information about the joint mechanics of the patient prior to the onset of OA. Although it would be inefficient to collect joint mechanics data on a large group of people and follow them for several years until some of them develop OA and eventually obtain a THA, this type of data would be extremely useful to place all the current studies into the proper context. Because a prospective study like this is difficult to conduct, I would encourage large orthopaedic biomechanics research labs that have been established for a long period of time, such as the BioMotion Lab at Stanford University or the Motion Analysis Lab at Rush University, to contact older patients who have been previously enrolled in movement studies as healthy controls. Several of these subjects have likely developed hip OA and could be re-tested before and after receiving a THA. Ultimately, research should be focused on identifying how movement and joint loading is influenced by hip OA and hopefully restored post hip replacement surgery. Controlling for patient- and surgery-specific parameters such as activity level and implant type will be important as medicine moves towards a more patient-specific approach.
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