Why isn't a shoulder problem the same as a knee problem?

Which shoulder parts usually become sore?

Four tendons hold the shoulder ball steady while your arm moves. Doctors call them the rotator cuff, and each tendon joins muscle to bone. Rotator cuff tendons can become sore, worn, or torn.

Cartilage, the smooth cover on the shoulder bones, can wear too. The joint capsule, a tough sleeve around the whole shoulder, can tighten and block motion. A doctor checks motion and strength to tell these problems apart.

Why can't knee results answer my shoulder question?

A knee carries your weight and bends mainly like a hinge. A shoulder carries less weight and turns in many directions. Its shallow socket also depends heavily on the rotator cuff. A treatment tested in knees may act differently in shoulders.

Most research on cartilage repair has focused on knees. Those results can't tell how much your shoulder soreness or motion will improve. Base your choice on your shoulder exam and the known limits.

Will an MRI tell me what hurts?

An MRI is a scan that shows tendons, cartilage, and bone. It may show a worn tendon that doesn't hurt. The doctor compares the scan with your strength and motion. That comparison tells whether the worn area may explain your soreness.

During the exam, you raise and turn the sore arm. You may also press against the doctor's hand to test strength. Sudden weakness after a fall brings more concern than slow soreness. The exam tells the doctor how much the MRI matters.

Sources

1. MACI (autologous cultured chondrocytes on a porcine collagen membrane, Vericel; STN BL 125603) IS an FDA-LICENSED cell therapy - and its approved indication is narrow and specific: repair of symptomatic, single or multiple FULL-THICKNESS cartilage defects OF THE KNEE, with or without bone involvement, in adults. It is not approved for osteoarthritis. The existence of one licensed cartilage cell therapy for focal defects is the sharpest available way to show what a licensed 'regeneration' product actually looks like, and how far it is from an injection for a worn joint.

   US Food and Drug Administration, Center for Biologics Evaluation and Research — MACI (autologous cultured chondrocytes on porcine collagen membrane). FDA, 2024.

2. The evidence overview underpinning the 2020 EULAR recommendations on intra-articular therapies pooled 29 quality-appraised systematic reviews. Hyaluronic acid showed a small effect on pain and function in KNEE OA but not in hip OA or shoulder capsulitis; intra-articular glucocorticoid showed small effects in knee OA and on function in hip OA and shoulder capsulitis; PRP showed benefit in knee OA but NOT in hip OA, and mesenchymal stem cells behaved similarly. Overall conclusion: most intra-articular therapies exert SMALL effects and are well tolerated.

   Rodriguez-García SC, et al. — Efficacy and safety of intra-articular therapies in rheumatic and musculoskeletal diseases: an overview of systematic reviews.. RMD open, 2021. DOI: 10.1136/rmdopen-2021-001658.

3. The Cochrane review of surgery for rotator cuff tears included nine trials and 1,007 participants; three of those trials (339 participants with MRI- or ultrasound-confirmed full-thickness tears) compared repair plus subacromial decompression followed by exercise against exercise ALONE. All included trials were at risk of bias on several criteria, most notably lack of participant and personnel blinding.

   Karjalainen TV, et al. — Surgery for rotator cuff tears.. Cochrane Database Syst Rev, 2019. DOI: 10.1002/14651858.CD013502.

4. A systematic review of seven case series (469 reverse total shoulder arthroplasties, mean age 71, mean 12-year follow-up) found weighted mean revision-free implant survivorship of 88% at 10 years, an overall complication rate of 36% and revision in 23% of patients - most often for infection (8%), instability (7%) and glenoid complications (3%) - alongside large functional gains (absolute Constant score 27 to 62).

   Biner M, et al. — Long-Term Outcomes Following Reverse Total Shoulder Arthroplasty: A Systematic Review with a Minimum Follow-Up of 10 Years.. JB JS Open Access, 2025. DOI: 10.2106/JBJS.OA.25.00025.

5. In a nationally representative Finnish population sample of 602 adults aged 41-76 who had bilateral 3-Tesla shoulder MRI regardless of symptoms, rotator cuff abnormalities were found in 98.7% of participants (25% tendinopathy, 62% partial-thickness tear, 11% full-thickness tear). Abnormalities were present in 96% of ASYMPTOMATIC shoulders. Only full-thickness tears were more common in symptomatic shoulders, and that difference all but disappeared after adjustment (absolute difference 0.8%, 95% CI -3.4% to 6.0%).

   Ibounig T, et al. — Incidental Rotator Cuff Abnormalities on Magnetic Resonance Imaging.. JAMA Intern Med, 2026. DOI: 10.1001/jamainternmed.2025.7903.