Heel Pad Sign

1. High-Yield Overview & Pathognomonic Essence

The heel pad sign represents an objective radiographic marker reflecting systemic soft tissue and fibro-adipose expansion:

• Morphologic & Radiographic Appearance: Diffuse, symmetrical thickening of the subcalcaneal soft tissue mantle on non-weightbearing lateral foot radiographs, defined by a shortest vertical distance between the inferior-most cortical margin of the calcaneal tuberosity and the plantar skin interface exceeding 23 mm (or greater than 21.5 mm in adult females and greater than 25 mm in non-obese populations).
• Pathognomonic Pathology: The classical peripheral musculoskeletal manifestation of adult growth hormone (GH) hypersecretion and elevated serum insulin-like growth factor 1 (IGF-1) in acromegaly, producing unconstrained fibroblastic proliferation, extracellular mucopolysaccharide accumulation, and diffuse fibro-adipose hypertrophy.
• Diagnostic Utility & Clinical Implication: Provides an accessible, non-invasive plain radiographic screening indicator that can unmask insidious pituitary disease years before overt systemic, cardiovascular, or craniomaxillofacial complications emerge; however, because post-treatment fibrous remodeling resolves very slowly, it remains an insensitive index of acute biochemical disease remission.

2. Diagnostic Sign Summary Matrix

3. Clinical Case Imaging & Standardized Figures

Figure 1 illustrates the classic radiographic manifestation of the heel pad sign on a standardized non-weightbearing lateral projection of the hindfoot. Measuring the shortest vertical distance between the inferior-most cortical margin of the calcaneal tuberosity and the outer plantar skin boundary reveals profound soft tissue widening measuring well in excess of the normal 21 mm threshold. The soft tissue envelope demonstrates uniform, diffuse expansion without focal lobular disruption, fluid layering, or architectural effacement. The underlying calcaneal trabecular architecture remains intact without cortical violation or fracture lines, highlighting that the expansion represents chronic endocrine-driven fibro-adipose deposition rather than acute traumatic edema or hematoma.

4. Pathophysiology & Biomechanical Mechanism

The heel pad sign is the peripheral soft tissue manifestation of prolonged, unmitigated somatotropic axis activation. In adult acromegaly, a benign somatotroph adenoma of the anterior pituitary gland autonomously hypersecretes growth hormone (GH), which stimulates hepatic and peripheral production of insulin-like growth factor 1 (IGF-1). Because physical growth plates (physes) undergo physiologic fusion at the cessation of puberty, longitudinal skeletal growth is no longer possible. Instead, persistent GH and IGF-1 elevation acts upon receptive mesenchymal cells, periosteal osteoblasts, synovial chondrocytes, and subcutaneous fibroblasts throughout acral anatomic sites.

The biochemical and cellular mechanisms driving calcaneal heel pad expansion comprise three coordinated pathophysiological processes:

• Extracellular Glycosaminoglycan Deposition & Interstitial Hydration: IGF-1 binds to transmembrane IGF-1 receptors on dermal and subcutaneous fibroblasts, activating intracellular MAPK and PI3K-Akt signaling cascades. This markedly upregulates the transcription and secretion of core extracellular matrix proteins and negatively charged glycosaminoglycans (GAGs), principally hyaluronic acid and chondroitin sulfate. Owing to their high polyanionic charge density, GAGs avidly bind mobile cations and water molecules, creating high osmotic swelling pressure and dramatic hydrophilic expansion within the interstitial matrix.
• Fibro-Adipose Hyperplasia & Architecture Maintenance: The calcaneal fat pad is a specialized mechanical organ engineered for shock attenuation, composed of enclosed hydraulic microchambers and macrochambers bounded by dense, spiraling fibro-elastic septae anchored from the calcaneal periosteum to the deep reticular dermis. Under prolonged GH stimulation, adipocytes undergo both hypertrophy and preadipocyte recruitment (hyperplasia), while the fibrous septae undergo thickened collagenous remodeling. Unlike traumatic septal rupture or degenerative fat pad atrophy, the acromegalic heel pad preserves its internal compartmentalized structural architecture while its total tissue volume expands symmetrically.
• Periosteal Appositional Hyperostosis & Enthesopathy: GH and IGF-1 directly stimulate osteoblasts within the vascular periosteum, initiating appositional new bone formation. At high-stress mechanical attachment sites, traction enthesophytes develop at the calcaneal tuberosity along the origin of the central plantar aponeurosis and the insertion of the Achilles tendon. Concurrent subchondral bone remodeling and cartilage hypertrophy across adjacent tarsal articulations alter biomechanical force transmission during gait.

From a biomechanical standpoint, the hypertrophied acromegalic heel pad exhibits altered visco-elastic damping properties. Although tissue volume is substantially expanded, collagen cross-linking and proteoglycan overloading increase internal matrix stiffness. As a result, energy dissipation during initial ground contact is impaired, transferring increased repetitive impact stresses across the subtalar joint, midtarsal joints, and metatarsophalangeal articulations, accelerating secondary degenerative osteoarthritis.

5. Modality-Specific Imaging Findings

Plain Radiography

Non-weightbearing lateral radiography of the foot remains the historic benchmark and primary screening tool for measuring heel pad thickness. Standardized acquisition mandates a 40-inch (100 cm) source-to-image distance (SID) centered over the hindfoot to avoid geometric beam divergence and false magnification. Measurement is performed by identifying the shortest vertical line connecting the inferior-most cortical contour of the calcaneus to the radiolucent skin margin on the plantar aspect of the heel.

Normal heel pad thickness averages 17.8 mm in healthy females and 19.5 mm in healthy males. Radiographic heel pad thickness exceeding 21.5 mm in adult women and 23.0 mm in adult men warrants suspicion, while values exceeding 25.0 mm in non-obese individuals are highly specific for acromegaly. Secondary osseous findings readily visible on plain radiographs include prominent plantar calcaneal spurs, hypertrophic Achilles enthesophytes, terminal tuft mushrooming of the distal phalanges (“spade-like” phalanges), widening of the joint spaces secondary to chondral hyperplasia in early stages, and prominent cortical hyperostosis across the metatarsal shafts.

Computed Tomography

Computed tomography (CT) provides multiplanar volumetric reconstructions that eliminate optical projection parallax and magnification artifacts inherent to plain films. On sagittal and coronal high-resolution CT reconstructions, heel pad thickness can be measured with sub-millimeter precision. The subcalcaneal soft tissues demonstrate homogeneous low attenuation characteristic of mature adipose tissue (-80 to -120 Hounsfield units), interspersed with delicate, regular soft tissue attenuation bands (+10 to +30 HU) representing intact fibro-elastic septae.

CT excels at evaluating the underlying osseous framework, confirming the absence of subtle calcaneal stress fractures or cortical erosions, detailing the exact morphology and size of plantar calcaneal spurs, and characterizing secondary osteoarthritic joint space narrowing, subchondral sclerosis, and osteophytosis in advanced acromegalic arthropathy.

Magnetic Resonance Imaging & Ultrasound

Magnetic resonance imaging (MRI) provides superior contrast resolution of the soft tissue compartments of the hindfoot. In patients with acromegaly, sagittal T1-weighted and T2-weighted fast spin-echo images demonstrate marked expansion of the subcutaneous fat pad with preservation of normal adipose signal intensity: high signal on non-fat-suppressed T1 sequences and intermediate signal on T2 sequences. Short tau inversion recovery (STIR) and T2-weighted fat-saturated sequences demonstrate complete absence of abnormal fluid signal, hyperintensity, or diffuse reticulation within the fat pad, definitively ruling out acute traumatic contusion, inflammatory cellulitis, or systemic interstitial edema. The proximal plantar aponeurosis typically maintains a normal thickness of less than 4.0 mm, confirming that the expansion is not driven by localized plantar fasciitis.

High-resolution musculoskeletal ultrasound (7.5 to 15 MHz linear array) represents a rapid, cost-effective, radiation-free diagnostic modality. With the patient prone and the foot hanging freely in a non-weightbearing state, the transducer is positioned longitudinally along the plantar midline of the calcaneal tuberosity without applying downward transducer pressure. Ultrasound directly measures the uncompressed vertical distance between the bright hyperechoic calcaneal bony reflection and the superficial epidermal margin. Ultrasound demonstrates intact, hyperechoic fibro-elastic septal architecture bounding hypoechoic fat lobules. Color Doppler evaluation reveals no internal hypervascularity, differentiating acromegalic expansion from active inflammatory enthesopathies or soft tissue neoplasms.

6. Diagnostic Utility, Sensitivity & Specificity

The diagnostic accuracy of heel pad thickness as a surrogate marker for acromegaly has been extensively validated in historic and contemporary cohorts:

• Sensitivity: Across landmark radiographic series (Steinbach & Russell 1964; Kho et al. 1970; MacSweeney et al. 1990), using a standard threshold of > 23 mm yields a diagnostic sensitivity ranging between 80% and 85% for acromegalic patients.
• Specificity: When adjusted for patient sex and body mass, specificity reaches 92% to 96%. Setting a conservative diagnostic cutoff of > 25 mm elevates specificity above 98% in non-obese clinical cohorts.
• Positive & Negative Predictive Values: In general outpatient populations with undifferentiated foot pain, the positive predictive value (PPV) is moderated by the prevalence of obesity and dependent edema. However, in patients presenting with concomitant acral symptoms (enlarging shoe or ring size, carpal tunnel syndrome, coarse facial skin, macroglossia), a heel pad exceeding 23 mm carries a PPV surpassing 90%, necessitating immediate endocrine referral. The negative predictive value (NPV) is approximately 88%, as a normal heel pad measurement does not completely exclude early or mild biochemical acromegaly.

Several technical and physiological confounding factors influence measurement precision and must be controlled:

• Patient Body Weight and Body Mass Index (BMI): Body weight exerts a direct physiological influence on soft tissue thickness. Kho et al. established that heel pad thickness increases by approximately 1 mm for every 10 to 15 kg increase in body weight. In morbidly obese individuals, heel pad thickness can exceed 23 mm purely secondary to adipose loading, requiring correlation with biochemical assays.
• Weightbearing vs Non-weightbearing Radiography: Upright weightbearing compresses the hydraulic chambers of the fat pad, forcing the soft tissues to spread laterally and reducing the vertical measurement by 2 to 4 mm. Diagnostic thresholds are strictly standardized on non-weightbearing lateral projections.
• Geometric Magnification & Source-to-Image Distance (SID): Standard tables are calibrated at a 40-inch (100 cm) SID. Reducing the focal distance to 36 inches or improper positioning of the hindfoot away from the detector amplifies geometric divergence, falsely increasing apparent soft tissue thickness by 1.5 to 2.5 mm.

7. Differential Diagnosis & Radiographic Mimics

8. Clinical Significance & Management Implications

The clinical identification of an enlarged calcaneal heel pad carries paramount diagnostic significance due to the notoriously protracted diagnostic latency of acromegaly. On average, patients experience progressive clinical symptoms for 5 to 10 years before definitive diagnosis, during which time irreversible cardiovascular hypertrophy, bitemporal hemianopsia, obstructive sleep apnea, diabetes mellitus, and debilitating degenerative arthropathy develop. Because patients frequently seek medical attention for non-specific foot pain, changing shoe sizes, or refractory carpal tunnel syndrome, orthopaedic surgeons, radiologists, and podiatric specialists serve as frontline diagnosticians.

Discovery of a heel pad exceeding 23 mm on plain radiographs in the absence of acute trauma or massive obesity should immediately prompt targeted clinical history-taking, physical examination of the hands and craniomaxillofacial structures, and formal endocrine screening. The mandatory biochemical initial step is obtaining an age- and sex-adjusted serum IGF-1 level. If elevated, the diagnosis is confirmed by an oral glucose tolerance test (OGTT) demonstrating failure of serum GH suppression below 1 mcg/L (or below 0.4 mcg/L on modern ultrasensitive assays), followed by gadolinium-enhanced pituitary MRI to localize the somatotroph adenoma.

Regarding longitudinal management and post-treatment monitoring, landmark investigations by MacSweeney et al. (1990) and Paisey et al. (1984) established that heel pad thickness is an insensitive and unreliable parameter for evaluating therapeutic response. Following successful transsphenoidal adenoma resection or pharmacotherapy with somatostatin receptor ligands (octreotide, lanreotide), serum GH and IGF-1 levels normalize within days to weeks. In contrast, the dense fibrous matrix remodeling, proteoglycan deposition, and adipose hyperplasia in the heel pad regress sluggishly over years, and the soft tissues rarely return to normal baseline thickness. Consequently, serial plain radiographs or sonographic heel pad measurements must not be utilized to monitor biochemical disease activity or declare endocrinologic remission; laboratory assays of serum IGF-1 remain the sole gold standard.

From an orthopaedic management perspective, acromegalic patients with heel pad thickening and altered gait biomechanics benefit from conservative measures, including custom total-contact orthotics with deep heel cups, shock-absorbing viscoelastic materials, and footwear with rocker-bottom soles to diminish impact shocks across the hindfoot and midfoot. If surgical intervention is contemplated for secondary peripheral nerve compressions (such as tarsal tunnel release or carpal tunnel release) or advanced joint arthroplasty, preoperative multidisciplinary clearance is essential to manage potential difficult intubation arising from macroglossia and vocal cord hypertrophy, as well as occult cardiomyopathy and sleep apnea.

9. Master Class Pro-Tip

Master Class Pro-Tip: When evaluating heel pad thickness on a lateral foot radiograph, verify two critical technical variables before suspecting endocrine pathology: the focus-to-film distance (SID) and whether the projection is non-weightbearing. A short focal distance exaggerates geometric magnification, falsely inflating heel pad measurements by 2 to 3 mm, while weightbearing compresses and spreads the fibro-adipose chambers, masking true soft tissue expansion. If an uncompressed lateral radiograph at 40-inch SID confirms heel pad thickness exceeding 23 mm in a non-obese patient without acute trauma, inspect the distal phalanges for spade-like tufting and examine the patient’s hands for carpal tunnel signs. Never order serial lateral radiographs to monitor response to pituitary surgery or medical therapy: fibrous matrix remodeling lags behind endocrine remission by years, making serum IGF-1 the only valid benchmark of disease control.

Master Class Radiological Pearl: Calcaneal Soft Tissue Morphometry and Endocrine Screening

10. Evidence-Based Synthesis

11. References

1. Steinbach HL, Russell W. Measurement of the Heel-Pad as an Aid to Diagnosis of Acromegaly. Radiology. 1964;82(3):418-423. DOI [Level III: Diagnostic Cohort Study]
2. Kho KM, Wright AD, Doyle FH. Heel pad thickness in acromegaly. Br J Radiol. 1970;43(506):119-125. DOI [Level III: Case-Control Diagnostic Study]
3. Paisey R, Jeans WD, Hartog M. Is soft tissue radiology useful in acromegaly? Br J Radiol. 1984;57(679):561-564. DOI [Level III: Retrospective Comparative Cohort Study]
4. Gooding GA, Stess RM, Graf PM, Moss AA, Lorenzi M. Heel pad thickness: determination by high-resolution ultrasonography. J Ultrasound Med. 1985;4(4):173-174. DOI [Level II: Prospective Diagnostic Validation Study]
5. MacSweeney JE, Baxter MA, Joplin GF. Heel pad thickness is an insensitive index of biochemical remission in acromegaly. Clin Radiol. 1990;42(5):348-350. DOI [Level IV: Longitudinal Comparative Cohort Study]
6. Katznelson L, Laws ER, Melmed S, et al. Acromegaly: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2014;99(11):3933-3951. DOI [Level V: Clinical Practice Guideline]
7. Ozturk GB, Gogus F, Bolayir B, et al. The evaluation of the tendon and muscle changes of lower extremity in patients with acromegaly. Pituitary. 2020;23(4):393-402. DOI [Level III: Prospective Controlled Cross-Sectional Study]
8. Bonarigo E, Easow A, Cohen S, et al. Retrospective Review Evaluating the Relationship Between Radiographic Calcaneal Fat Pad Thickness and Plantar Fasciitis. J Am Podiatr Med Assoc. 2025;115(4):23-126. DOI [Level IV: Retrospective Comparative Series]