Background: Postoperative rehabilitation following Total Knee or Hip arthroplasty (TKA, THA respectively) for end-stage osteoarthritis is frequently characterized by oxidative stress and chronic-inflammation-induced muscle atrophy. This study investigated the efficacy of a milk protein concentrate supplement (MCPS) on oxidative stress, inflammation markers, and functional regains in patients undergoing TKA or THA. Methods: 88 participants (aged 55–80 years) were allocated to either an Intervention Group (IG, n = 44), receiving the MPCS, or a Control Group (CG, n = 44), following conventional nutrition for 15 weeks. Appendicular skeletal muscle mass (ASMM) was measured using bioelectrical impedance analysis and functionality through handgrip strength, gait speed, and static balance. 8-Isoprostane levels were quantified in plasma samples using the Enzyme-Linked Immunosorbent Assay(ELISA) method. C Reactive Protein (CRP) levels in serum specimens were measured. Data analysis was conducted, with adjustments made for age, gender, and comorbidities. Results: The IG demonstrated a significant increase in ASMM (Adj. mean change, Δ = +2.34 kg, 95% CI: 1.99 to 2.69, p < 0.001) and ASMM Index (Δ = +0.82 kg/m², 95% CI: 0.64 to 1.00, p < 0.001) compared to the CG. Functional measurements also showed significant improvements in the IG, including Handgrip Strength (Δ = +4.40 kg, p < 0.001), Gait Speed (Δ = +0.23 m/s, p < 0.001), and the 2-Minute Walk Test (Δ = +12.02 m, p = 0.026). Regarding biochemical markers, the IG showed a significant reduction in plasma F2-isoprostane levels (Δ = −29.19, p < 0.001), CRP levels (Δ = −0.69 mg/L, p = 0.004), and PTH levels (Δ = −27.41 pg/mL, p < 0.001). A negative association between lipid peroxidation (8-isoprostanes) and ASMM was confirmed. Conclusions: Structural nutritional intervention can effectively mitigate catabolic stress triggered by surgical treatment. Implementing such strategies into orthopedic care offers a practical approach to treat challenges often associated with postoperative muscle loss.

Background: Skeletal muscle impairment is common in older adults with end-stage knee and hip osteoarthritis (OA) and contributes to disability and functional decline. Τhe relative contributions of muscle mass, muscle strength, systemic inflammation and oxidative stress to sarcopenia phenotypes in this population remain incompletely understood. This study investigated the associations between muscle-related parameters, inflammatory and oxidative stress biomarkers, and sarcopenia phenotypes in older adults with end-stage OA. Methods: This cross-sectional study included 88 older adults scheduled for primary total knee (n = 56) or total hip arthroplasty (n = 32). Body composition, muscle strength, physical performance, and biochemical markers were assessed using standardized procedures. Systemic inflammation was evaluated using C-reactive protein (CRP) and the neutrophil-to-lymphocyte ratio (NLR), whereas oxidative stress was assessed by urinary F₂-isoprostanes. Sarcopenia status was categorized according to the European Working Group on Sarcopenia in Older People 2 (EWGSOP2) criteria. Results: Overall, 29.5% of participants were classified as having probable/confirmed sarcopenia phenotypes, with similar prevalence in knee and hip OA. Sarcopenic participants exhibited significantly lower appendicular skeletal muscle mass (ASMM), reduced handgrip strength, slower gait speed, poorer physical performance and higher body fat percentage than those without sarcopenia. Muscle strength demonstrated stronger associations with physical performance than muscle mass and remained an independent predictor of Short Physical Performance Battery (SPPB) score after adjustment for age, gender and muscle mass. Higher urinary F₂-isoprostane concentrations were independently associated with lower ASMM after adjustment for demographic characteristics and inflammatory biomarkers, whereas CRP and NLR were not independently associated with muscle-related outcomes in the fully adjusted models. Conclusions: Nearly one-third (29.5%) of older adults with end-stage knee and hip OA were classified as having probable or confirmed sarcopenia according to the EWGSOP2 criteria. Oxidative stress was independently associated with skeletal muscle mass, whereas CRP and NLR were not independently associated with muscle-related outcomes. These findings highlight the relevance of muscle strength and oxidative stress in older adults with end-stage OA and warrant further longitudinal investigation.