Total hip replacement surgery, also known as total hip arthroplasty (THA), is becoming increasingly common. Because of this, ensuring patients have the best possible results after their operation is a key goal. A frequent issue for individuals with hip problems is the co-occurrence of low back pain.

To measure how well a patient is doing after surgery, healthcare providers often use patient-reported outcome measures (PROMs). These are questionnaires where patients describe their own pain levels, ability to function, and overall well-being. While useful, these self-reports can have limitations. Therefore, researchers are interested in physical measures—objective tests conducted by a professional, such as measuring walking speed—to see if they provide valuable information.

This research aimed to review existing studies to see if there is a connection, or association, between these two types of measurements after a person has had a hip replacement. Specifically, it looked for two things:

1. The association between physical measurements and what patients report on questionnaires. The physical measurements studied were "spinopelvic alignment" (the posture and positioning of the spine relative to the pelvis) and tests of physical functioning (like walking or climbing stairs).
2. The association between these physical measurements and the presence of low back pain.

To do this, a systematic review was performed. This involved a structured and comprehensive search of scientific databases, unpublished research (known as grey literature), and relevant academic journals for studies published up to May 31, 2024. The review included studies that looked at patients at a single point in time (cross-sectional) as well as studies that followed patients over a period of time (longitudinal). Two reviewers independently selected the studies, collected the data, and assessed each study's quality by determining its risk of bias, which is the likelihood that its design could lead to inaccurate conclusions.

Because the methods used in the included studies were very different from one another, their results could not be combined statistically. Instead, the findings were summarized in a descriptive way, called a narrative synthesis. The overall quality of the evidence was rated using a system known as GRADE.

The review included 51 studies in total. Of these, eight were judged to have a low risk of bias (higher quality), 24 had a moderate risk, and 19 had a high risk of bias (lower quality).

Very few studies (only six) looked at the alignment of the spine and pelvis, so the main summary focused on physical tests of a person's ability to function. The review found very low-quality evidence suggesting there is no connection between certain physical tests and what patients report.

The specific findings, all based on very low-quality evidence, were:

• In the short-term period after surgery, there was no association found between a patient's walking speed and their self-reported level of function.
• In the medium-term period, there was no association between the amount of light-intensity physical activity a patient did and the amount of physical activity they reported doing.
• Looking at changes over a short-term period, improvements in two specific tests—the "Timed-Up-and-Go" test (which measures the time it takes to stand up, walk a short distance, and sit back down) and the "Stair-Climb-Test"—were not associated with changes in a patient's self-reported pain or function.
• Looking at changes over a medium-term period, improvements in the Stair-Climb-Test were not associated with changes in self-reported function.

Other findings across the various studies were inconsistent. Additionally, there were only three studies that examined low back pain, and they were too different from each other to be summarized together.

In conclusion, the available evidence is of very low quality but suggests there is no association between objective physical measures of functioning and the outcomes that patients report themselves after hip replacement surgery. The number of studies looking at low back pain was very limited. The significant differences between the existing studies and the small number of high-quality studies indicate a need for future research to better understand the relationship between these different types of measures.