Tendinitis: What Could Be Causing It

What Tendinitis Actually Is

Tendinitis refers to inflammation of a tendon, the fibrous cord connecting muscle to bone. The term is sometimes used interchangeably with "tendinopathy," though the distinction matters clinically. True tendinitis involves an acute inflammatory response, while tendinopathy describes chronic degenerative changes in the tendon's collagen matrix without significant inflammation [1].

Acute vs. Chronic Tendon Disease

Acute tendinitis typically develops over days to weeks following a new activity or sudden increase in load. Histologically, the tendon shows inflammatory cell infiltration and edema. By contrast, chronic tendinopathy (lasting more than 6 to 12 weeks) shows disorganized collagen fibers, neovascularization, and minimal inflammatory cells on biopsy [2]. A 2019 review in the British Journal of Sports Medicine confirmed that most patients presenting with "tendinitis" symptoms beyond 6 weeks actually have degenerative tendinopathy rather than active inflammation [3].

Where Tendinitis Strikes Most Often

The tendons most frequently affected are the rotator cuff (supraspinatus), lateral epicondyle (tennis elbow), Achilles, patellar, and wrist extensors. The rotator cuff alone accounts for roughly 4.5 million physician visits per year in the United States [4]. Achilles tendinopathy affects 5.9% of sedentary individuals and up to 50% of competitive runners at some point in their careers, according to data published in the Journal of Bone and Joint Surgery [5].

Overuse and Mechanical Causes

Repetitive mechanical loading is the single most common trigger for tendinitis. When a tendon is stressed beyond its capacity to repair between loading cycles, microdamage accumulates faster than the body can remodel [6].

Repetitive Motion and Training Errors

Office workers performing repetitive wrist extension, assembly-line workers, and musicians are at elevated risk for wrist and elbow tendinitis. In athletes, training errors account for 60% to 80% of overuse tendon injuries, according to a systematic review published in Sports Medicine [7]. The typical pattern involves a rapid increase in training volume or intensity (more than 10% per week), insufficient recovery time, or a sudden change in surface or equipment.

Biomechanical Factors

Structural malalignment can redistribute load onto vulnerable tendons. Overpronation of the foot increases strain on the Achilles tendon and tibialis posterior. A leg-length discrepancy as small as 5 mm can alter gait mechanics enough to overload the patellar tendon on the longer side [8]. Muscle imbalance, particularly weak hip abductors in runners, has been linked to increased patellar and Achilles tendon loading in biomechanical studies [9].

Occupational Risk

A large Finnish cohort study (N=6,943) published in Scandinavian Journal of Work, Environment & Health found that workers performing repetitive hand-intensive tasks had a 2.6-fold higher incidence of lateral epicondylitis compared to workers in non-repetitive roles [10]. Jobs requiring sustained overhead arm postures (painters, electricians) carry elevated risk for rotator cuff tendinitis.

Age-Related Changes

Tendons lose water content, cellularity, and organized collagen cross-linking after age 30 to 35. This is not a disease process. It is normal biology.

Collagen Degradation Over Time

Type I collagen turnover slows with age. A study using carbon-14 dating of Achilles tendon collagen, published in The Journal of Experimental Medicine, found that the core of adult tendons contains collagen synthesized during skeletal growth, with minimal turnover after age 17 [11]. This means adult tendons have a limited capacity to replace damaged fibers. The clinical result: tendons become stiffer, less elastic, and more prone to microtearing under loads they previously tolerated.

Reduced Vascularity

The Achilles tendon's watershed zone (2 to 6 cm proximal to insertion) receives limited blood supply. Aging further reduces perfusion in this region, making it the most common site of Achilles tendinopathy and rupture [12]. The rotator cuff's "critical zone" near the supraspinatus insertion has a similar vascular vulnerability, which is why rotator cuff tendinitis peaks after age 40.

Systemic and Metabolic Causes

Not all tendinitis comes from overuse. Several systemic conditions predispose tendons to inflammation and degeneration, and these causes are frequently underdiagnosed.

Diabetes and Metabolic Syndrome

Diabetes is one of the strongest systemic risk factors for tendinopathy. Advanced glycation end-products (AGEs) accumulate in tendon collagen, altering its mechanical properties and triggering low-grade inflammation. A meta-analysis of 31 studies (N=9,082) published in Diabetes Care found that individuals with type 2 diabetes had a 3.7-fold increased risk of rotator cuff tendinopathy and a 2.9-fold increased risk of Achilles tendinopathy compared to non-diabetic controls [13]. HbA1c levels above 7% correlated with greater tendon thickness and more severe structural changes on ultrasound.

Thyroid Disorders

Both hypothyroidism and hyperthyroidism alter tendon metabolism. Hypothyroidism reduces proteoglycan synthesis in tendons, leading to increased stiffness and susceptibility to microdamage. A population-based study in Annals of the Rheumatic Diseases (N=689,572) reported that patients with hypothyroidism had a 1.4-fold higher incidence of rotator cuff disease [14].

Inflammatory and Autoimmune Conditions

Rheumatoid arthritis (RA), psoriatic arthritis, and spondyloarthropathies all involve tenosynovitis as part of their disease spectrum. In psoriatic arthritis, enthesitis (inflammation at the tendon-bone junction) is a hallmark feature, present in up to 35% of patients at diagnosis [15]. Recognizing tendinitis as a manifestation of systemic disease rather than an isolated mechanical problem changes management entirely: these patients need disease-modifying therapy, not just ice and rest.

Elevated Uric Acid and Gout

Monosodium urate crystals can deposit in and around tendons, causing acute or chronic tendinitis. The Achilles tendon is the most commonly affected site. A study in Arthritis & Rheumatology demonstrated urate deposits on dual-energy CT in 36% of patients with gout, even in those without a history of tendon symptoms [16].

Medication-Induced Tendinitis

Certain medications directly damage tendon structure. Identifying these is important because the treatment is discontinuation, not physical therapy.

Fluoroquinolones

Fluoroquinolone antibiotics (ciprofloxacin, levofloxacin, moxifloxacin) carry an FDA black-box warning for tendinitis and tendon rupture [17]. The risk is highest within the first 48 hours of use but persists for up to 6 months after discontinuation. A large population-based study published in BMJ (N=6,354 cases) found that fluoroquinolone use increased Achilles tendon rupture risk 3.2-fold in the general population and 6.2-fold in patients over age 60 [18]. Concurrent corticosteroid use multiplied the risk further.

Statins and Aromatase Inhibitors

Statins are associated with tendinopathy in case series, though the absolute risk is low. The mechanism may involve impaired cholesterol synthesis in tenocytes. Aromatase inhibitors used in breast cancer treatment (anastrozole, letrozole) cause musculoskeletal symptoms including tendinitis in up to 50% of patients, likely through estrogen depletion effects on tendon collagen [19].

Corticosteroids

Paradoxically, while corticosteroid injections are used to treat tendinitis, prolonged systemic corticosteroid use weakens tendons. Chronic prednisone use inhibits collagen synthesis and increases the risk of spontaneous tendon rupture, particularly of the Achilles and quadriceps tendons [20].

How Tendinitis Is Diagnosed

Diagnosis is primarily clinical. Imaging plays a supporting role.

Physical Examination

The clinician assesses for localized tenderness directly over the affected tendon, pain with resisted contraction of the associated muscle, and sometimes palpable thickening or crepitus. Specific provocative tests guide the diagnosis: the Neer and Hawkins tests for rotator cuff impingement, Cozen's test for lateral epicondylitis, and the Thompson test to rule out Achilles rupture [21].

Imaging

Ultrasound is the first-line imaging modality for tendinopathy. It detects tendon thickening, hypoechoic regions (indicating degeneration), neovascularization, and partial tears with sensitivity comparable to MRI for most superficial tendons [22]. MRI is reserved for cases where surgery is being considered, when deep tendons (rotator cuff) need detailed evaluation, or when the diagnosis is uncertain. Plain X-rays are useful to identify calcific tendinitis or bony spurs contributing to impingement.

Laboratory Testing

Routine blood work is not required for straightforward overuse tendinitis. However, if tendinitis is bilateral, affects multiple sites, or occurs in a patient under 30 without a clear mechanical cause, screening for rheumatoid factor, HLA-B27, uric acid, thyroid function, and HbA1c can uncover a systemic driver [23].

Treatment Options

Treatment follows a stepwise approach: load management first, then targeted rehabilitation, with procedures reserved for refractory cases.

Rest and Load Modification

Complete immobilization is counterproductive for most tendinopathies. Relative rest (avoiding aggravating activities while maintaining general movement) preserves tendon blood flow and prevents deconditioning [24]. The goal is to find a loading level that the tendon tolerates without worsening symptoms. For Achilles tendinopathy, this might mean switching from running to cycling temporarily.

NSAIDs and Analgesia

Oral NSAIDs (ibuprofen 400 to 600 mg three times daily, naproxen 500 mg twice daily) provide short-term pain relief in acute tendinitis. A Cochrane review of 15 trials found that topical NSAIDs were as effective as oral forms for superficial tendinopathies like lateral epicondylitis, with fewer gastrointestinal side effects [25]. NSAIDs are most useful in the first 7 to 14 days. Their benefit in chronic tendinopathy (where inflammation is minimal) is limited.

Eccentric Exercise and Physical Therapy

Eccentric loading (lengthening the muscle-tendon unit under tension) is the most evidence-supported rehabilitation approach for tendinopathy. The Alfredson eccentric heel-drop protocol for Achilles tendinopathy showed 89% satisfaction at 5-year follow-up in the original Swedish cohort [26]. For patellar tendinopathy, heavy slow resistance training (3 sets of 15 repetitions, progressing to 6 repetitions, three times weekly for 12 weeks) produced equivalent outcomes in a randomized trial published in the American Journal of Sports Medicine [27].

Corticosteroid Injections

Corticosteroid injections provide rapid pain relief (within 1 to 2 weeks) but carry risks. A systematic review in The Lancet (46 trials, N=2,672) found that corticosteroid injections improved short-term outcomes (at 6 weeks) but worsened long-term outcomes (at 6 to 12 months) compared to physical therapy for lateral epicondylitis [28]. Repeated injections weaken tendon structure and increase rupture risk. Most guidelines now recommend limiting injections to 2 to 3 per site and spacing them at least 6 weeks apart.

PRP and Emerging Therapies

Platelet-rich plasma (PRP) injections have shown mixed results. A meta-analysis in the British Journal of Sports Medicine (18 RCTs, N=1,066) found modest benefit for lateral epicondylitis and patellar tendinopathy but no clear advantage over placebo for Achilles tendinopathy [29]. Extracorporeal shockwave therapy (ESWT) has moderate evidence for calcific rotator cuff tendinitis, with a meta-analysis showing 73% improvement in calcification resorption compared to 28% with sham treatment [30].

Surgery

Surgery is reserved for cases failing 6 to 12 months of conservative management. Procedures include debridement of degenerative tissue, repair of partial tears, and decompression of impinging structures. For rotator cuff tendinopathy, arthroscopic subacromial decompression showed similar long-term outcomes to structured physical therapy in the CSAW trial (N=313) published in The Lancet, raising questions about the value of surgical intervention for many patients [31].

When to Worry About Tendinitis

Most tendinitis resolves with conservative treatment. Certain features, however, warrant urgent evaluation.

Red Flags

Sudden, severe pain with a popping sensation suggests tendon rupture, not tendinitis. Inability to bear weight (Achilles) or lift the arm above shoulder height (rotator cuff) after an acute event requires same-day imaging. Night pain unrelated to position may indicate a condition beyond simple tendinopathy, including infection or tumor, and should prompt further workup [32].

Persistent or Recurrent Cases

Tendinitis lasting beyond 6 weeks despite appropriate load modification and rehabilitation may represent chronic tendinopathy requiring a modified treatment approach (heavy slow resistance rather than simple rest) or may signal an undiagnosed systemic condition. Bilateral tendinitis, tendinitis at multiple sites, or tendinitis in a young patient without overuse history should trigger screening for inflammatory, metabolic, or medication-related causes.

The "It Will Just Go Away" Problem

A retrospective cohort study in British Journal of General Practice found that 30% of patients with Achilles tendinopathy still reported symptoms at 5 years, and those who delayed treatment beyond 3 months had worse long-term outcomes [33]. Early intervention with structured rehabilitation, not prolonged rest, produces the best results. The clinical directive: if tendon pain persists beyond 2 weeks of simple activity modification, begin a supervised eccentric exercise program rather than waiting.