This observational study aims to better understand how knee pain and knee-specific body perception are related over time after total knee arthroplasty (knee replacement) for knee osteoarthritis. Although most people improve after surgery, some continue to experience ongoing pain. Disturbed body perception such as feeling that the knee is less clear, less familiar, or difficult to "sense" or "control" as usual-has been linked to chronic musculoskeletal pain, but the timing of its relationship with postoperative pain after knee replacement is not well understood.

Adults undergoing primary total knee arthroplasty at Fukuoka Orthopaedic Hospital will be invited to participate. Participants will be asked to complete brief self-report questionnaires about (1) knee pain intensity during movement, (2) knee pain intensity at rest, and (3) knee-specific body perception (how the knee feels and is perceived). Pain intensity will be recorded using 100-mm visual analogue scales, and knee-specific body perception will be assessed using a validated questionnaire. Questionnaires will be completed before surgery (within one week prior) and at 1, 3, 6, and 12 months after surgery.

All surgical and rehabilitation care will be provided as part of usual clinical practice. Participation in this study involves completing questionnaires only and does not change clinical care. The study will examine whether within-person changes in knee body perception are associated with later changes in pain intensity across the first postoperative year, while also accounting for stable differences between individuals.

Information from this study may help identify potentially modifiable factors associated with persistent pain after knee replacement and may inform future research on rehabilitation strategies that target body perception and related sensorimotor processes. Study data will be handled confidentially in accordance with institutional policies, and analyses will be conducted on de-identified data whenever possible.

Knee osteoarthritis is a common cause of pain and disability, and total knee arthroplasty (TKA) is frequently performed to improve symptoms and function. Although many people experience substantial improvement after surgery, a subset report ongoing pain during recovery. Disturbed body perception (e.g., reduced clarity, altered awareness, or a sense that the knee feels unfamiliar) has been associated with persistent musculoskeletal pain, but the temporal ordering of postoperative pain and knee-specific body perception after TKA remains unclear.

This observational cohort study follows adults undergoing primary TKA and examines longitudinal associations between pain intensity (during movement and at rest) and knee-specific body perception across the first postoperative year. Participants complete brief self-report questionnaires at preoperative baseline and at multiple postoperative follow-up time points through 12 months. Clinical care (surgery, analgesia, and rehabilitation) is provided as usual and is not altered by study participation; the study procedures involve completion of questionnaires only.

The primary objective is to evaluate whether within-person changes in knee-specific body perception are prospectively associated with later within-person changes in pain intensity over time, while accounting for stable between-person differences. Longitudinal analyses will use random-intercept cross-lagged panel modeling to separate trait-like, between-person stability from state-like, within-person fluctuations and to estimate autoregressive and cross-lagged associations across adjacent waves. Missing questionnaire data will be handled using full-information maximum likelihood under a missing-at-random assumption, and sensitivity analyses will be conducted using the complete-case sample to evaluate robustness. Descriptive summaries and correlations will be used to characterize the observed trajectories and concurrent associations across time points.

The study is conducted in accordance with institutional ethical approval and written informed consent. Data are managed confidentially in line with institutional policies, and analyses are performed on de-identified data whenever possible. Findings are intended to inform future research on potentially modifiable factors related to persistent pain after TKA and to support development of rehabilitation strategies that may incorporate body perception-related approaches.

Inclusion Criteria:

• undergoing primary TKA for knee OA
• ability to understand and complete study questionnaires
• provision of informed consent.

Exclusion Criteria:

• prior knee surgery on the affected knee
• significant neurological or cognitive impairment
• diagnosis of rheumatoid arthritis
• having undergone unicompartmental knee arthroplasty (UKA).

The study population consists of adults with knee osteoarthritis who are scheduled to undergo primary total knee arthroplasty at Fukuoka Orthopaedic Hospital. Participants are recruited consecutively from routine clinical practice and followed from the preoperative period through 12 months after surgery. Individuals are eligible if they can understand and complete study questionnaires and provide written informed consent. Key exclusions include prior surgery on the affected knee, significant neurological or cognitive impairment, rheumatoid arthritis, predominantly lateral compartment knee osteoarthritis, or planned unicompartmental knee arthroplasty.

De-identified individual participant data (IPD) are not planned to be shared publicly because the informed consent and institutional policies for this observational study did not include broad data-sharing provisions, and there is a risk of re-identification in a single-center dataset with detailed longitudinal measures. However, de-identified data may be considered for sharing on reasonable request for legitimate academic research, subject to review by the study investigators and approval under applicable ethics and data-use requirements.