Point of Care Diagnostic Ultrasound in the Evaluation of Patients with Patellar Tendinopathy

By Sarah L. Woelfel, MA, LAT, ATC and Adam B. Rosen, PhD, ATC

Point of care diagnostic ultrasound can be a useful tool for the evaluation, diagnosis, treatment, and risk identification of patellar tendinopathy. Measurements of the patellar tendon that may be useful include tendon thickness, stiffness, and biomarkers of tendon quality which can provide clinicians with a more comprehensive assessment of the histological and healing progress of the patellar tendon. However, for sports medicine clinicians, appropriate training to ensure reliability and proficiency is paramount. 

Patellar tendinopathy is a highly prevalent condition that affects individuals of all ages, compromising their abilities to comfortably perform activities of daily living, as well as other physical and recreational activities. While patellar tendinopathy can develop in the general population — one study from a Dutch general medical practice found that lower extremity tendinopathies have prevalence and incidence rates of 11.83 and 10.52 per 1000 person-years¹ – the condition is common among physically active individuals, particularly those  involved in running and jumping sports.^1–4 For example, one study showed prevalence of symptoms of patellar tendinopathy was 44.6% and 31.9% for volleyball and basketball athletes, respectively.⁵ In a recreational running population, patellar tendinopathy was one of the top 3 most common pathological conditions along with patellofemoral pain syndrome and iliotibial band friction syndrome.⁶

Documented risk factors for patellar tendinopathy include being male, higher body mass index, taller height, younger age, playing surfaces, increased amount of training — including jumping and running, and sports participation.^2,5,7 If mismanaged, patellar tendinopathy can lead to activity restrictions, lengthy therapeutic rehabilitation, and even the end of athletic careers.³

An umbrella term for tendon degeneration and inflammation, patellar tendinopathy can lead to daily aggravating pain that requires activity restrictions and long-term rehabilitation and care to treat and manage symptoms.³ Typically patients report pain that is localized to the anterior knee and can be related back to an increase in the frequency and intensity of physical activities that overload the patellar tendon; such overloading leads to micro-tears and the degeneration of tissue.³ Signs or symptoms used in the clinical diagnosis of patellar tendinopathy include

• pain localization at the inferior pole of the patella and inferior toward the tibial tuberosity along the patellar tendon,
• load-related pain that escalates with increased intensity or plyometric exercises that stress the patellar tendon,
• little to no pain when resting,
• decreased pain after warming-up and continuing exercise,
• then increased pain the day after exercise.⁴

Diagnosis of patellar tendinopathy is typically clinical, but imaging is common for confirmation and to eliminate potential differential diagnoses and assess tendon integrity.⁴

Common imaging for the patellar tendon is magnetic resonance imaging (MRI) or diagnostic ultrasound (DU) to assess the integrity of the tendon and surrounding structures of the knee.³ Currently, the accuracy for MRI in diagnosing patellar tendinopathy is approximately 70%, while the accuracy for correctly identifying patellar tendinopathy with DU is about 83%.⁸ Compared to MRI, ultrasound also can provide significant real-time advantages to the practicing clinician.

Diagnostic Ultrasound

Point-of-care DU has become increasingly popular due to its wide availability, cost, safety, speed, and accessibility of the procedure. With continued growth over the last few decades, DU at the point of care has the ability to evaluate tendons for elasticity, tissue stiffness and quality, and tendon health.⁹ Typically, studies comparing individuals with patellar tendinopathy with healthy individuals have found differences in ultrasonographic measures, depending on the method used and the location of interest. Different ultrasonographic methods (eg, compression elastography, shear-wave elastography, spatial frequency analysis, etc.) can be utilized to evaluate a variety of tendon properties including anthropometrics, tendon integrity, tissue quality, and stiffness of the patellar tendon. Diagnostic ultrasound can be useful in sports medicine settings for early diagnosis of patellar tendinopathy or tendon abnormalities and has the potential to be used during tendon evaluations as an objective outcome for evaluating the effectiveness of treatment and rehabilitation protocols for patellar tendinopathy.

Tendon Anthropometrics

Basic measurements that can be obtained with DU include tendon length, width, and thickness. Tendon length and width primarily provide more information regarding the basic structure of the patellar tendon, with the potential of observing obvious tendon abnormalities (Figure 1). Some abnormalities may include increased or decreased tendon length due to limb length or subject height.¹⁰ Other deformities that may be observed when measuring tendon length and width are obvious tears or partial tears in the tendon, but those abnormalities may require further evaluation via MRI.

Tendon thickness, meanwhile, may provide more information regarding the condition of the tendon that is clinically relevant. Pathological tendons present with tissue disorganization, which manifests upon evaluation and imaging as a thicker tendon compared to healthy tendons.¹¹ In addition, scanning the entire length of the patellar tendon may yield differences along the continuum of the patellar tendon, for example at the origin at the inferior pole of the patella or the insertion at the tibial tuberosity (Figure 2). One study that evaluated patellar tendons in athletes found that the proximal patellar tendon was significantly thicker in those with patellar tendinopathy.¹² While another study evaluated patellar tendons in beach volleyball players and noted that individuals with no tendinosis had an average patellar tendon thickness of 4.7/4.8 mm (dominant vs non-dominant knee) while symptomatic athletes demonstrated an average thickness of 7.6/7.0 mm( P <0.001 for dominant knee; P = 0.001 for non-dominant knee).¹³ Therefore thickness may be an important metric for clinicians to capture.

Ultrasound Elastography

Ultrasound elastography (USE), first introduced in the 1990’s by Ophir et al,¹⁴ has the ability to characterize tendon tissue and their response to stress.¹⁵ Elastography measures the elastic modulus to quantify tendon elasticity. Compared to other imaging options, this measure provides a wide variation of parameters that allows for greater discrimination between tissues and pathologies.¹⁵ Young’s elastic modulus, a common metric used to reflect the elastic properties of the tendon, can be estimated by calculating the slope of the elastic portion of the stress-strain curve.¹⁵ There are a few different categories of elastography, including compression elastography (CE) and sheer-wave elastography (SWE).

Compression Elastography

Compression elastography (CE) is characterized by the application of repeated manual compression on tissues by utilizing a hand-held US transducer to reproduce tissue displacements, or strain on the tendon, to exhibit relative elasticity.¹⁵ Using rhythmic and regular compressions of the area, the evaluator is able to target a specific area of interest to observe qualitative strain on the tissue. These strains are collected, and a specialized software is used to display a color-coded elastogram, typically displayed next to the conventional ultrasound image on the screen. In the color-coded elastogram, red frequently indicates soft (high-strain), blue indicates hard (low-strain), green and yellow indicate intermediate stiffness (medium-strain).¹⁶ More deformation is observable in softer tissues, which then experience larger strain than stiffer tissues which experience lesser strain.¹⁵ Successful and accurate measurements with CE depend on the depth of the targeted tissues and probe position, which make this technique more difficult to execute and operator-dependent.¹⁷ If the stress distribution within the tissue is not uniform or standardized during CE, this can affect the Young’s elastic modulus measurement that describes the elasticity of the tissue.¹⁵

Compression elastography has been proven to differentiate between normal and pathologic tissues. In a study comparing CE evaluation of symptomatic and asymptomatic patellar tendons in 75 athletes (average age 33 yrs), symptomatic tendons (n = 38) showed higher strain scores compared to asymptomatic patellar tendons.¹⁸ In addition, neovascularization was detected in 58% of the symptomatic tendons. Another study utilizing CE to investigate patellar tendons (N = 70 patellar tendons) in 35 volleyball athletes found that symptomatic patellar tendons were softer compared to asymptomatic.¹⁹

And according to Porta et al, CE showed good values for intra-observer and inter-observer agreement in patients with patellar tendinopathy and had an easy training curve.²⁰

Shear-wave Elastography

Shear-wave elastography (SWE) can also be utilized to characterize absolute tissue elasticity of soft tissues. SWE involves a perpendicular stress applied to the tissue which produces shear-waves. These waves are then measured with radiofrequency images to create a colored tissue displacement map that is used to calculate shear-wave velocity and shear modulus.¹⁷ Thus, we are allowed to assess the elastogram to evaluate the tendon tissue for elasticity in a more objective way compared to CE since there is no repeated manual compression utilized in this technique. The displacement will display different colors as noted above: red typically defines hard tissue consistency, blue as soft consistency, and green and yellow as in-between. Modifications on tissue elasticity can potentially be detected earlier than observable deformities with conventional ultrasound images most often termed B-mode images in the literature (Figure 3).¹⁵ Studies that investigated SWE demonstrated increased stiffness in symptomatic tendons and increased stiffness in the proximal and mid-patellar region.^21,22

Specifically for patellar tendinopathy, patients tend to have increased stiffness and have a significant correlation between the increased stiffness and intensity of pain and dysfunction.²³ This same study also demonstrated that symptomatic patellar tendons displayed higher shear modulus and were larger and stiffer in patients with proximal patellar tendinopathy.^23,24 It has also been suggested that SWE can aid in the prediction of impeding tendon failure, thus helping clinicians make informed clinical decisions in implementing preventative rehabilitation protocols.¹⁷ Hsiao et al discovered decreased elasticity (decreased elastic modulus) in the patellar tendon in an older age group (aged 60-70 years) compared to 2 younger age groups (20-30 and 50-60 years), thus suggesting the ability to detect aging tendons before visible abnormalities are observed using other imaging methods.²⁵

According to several studies, SWE has been shown to produce excellent intra-observer and inter-day reliability and moderate to excellent inter-observer reliability.^23,26,27 Consequently, SWE has been considered more objective, quantitative, and reproducible when compared to CE, since CE requires a certain level of replicability of manual compression with measurements.^15,17 Some limitations of SWE involve sensitivity to angle, pressure, and orientation of the probe along with shallow depths of penetration or region of interests, depending on the specific transducer or ultrasound machine.

Spatial Frequency Analysis

Spatial frequency analysis (SFA) is a specific ultrasound method that can characterize musculoskeletal tissue organization from ultrasound images and distinguish between pathological and healthy tendon tissue.^28–31 SFA is relatively new and can potentially be used for the diagnosis of patellar tendon injury or as a helpful measure for tracking tendon tissue adaptation during the rehabilitation process post-injury.^28–31 Analyzing B-mode images of the patellar tendon will display a speckle pattern in which the brightness of the image depends on the spatial distribution of the acoustic scatters within the tendon.³² The structure and alignment of the tendon tissues determines the characteristic pattern and intensity.^32,33 One study suggests that disruptions to the speckle pattern occurs with tendon damage and can be observed in patients with tendinopathy and was successful in classifying tendon tissues.³³ Typically higher SFA translates to a well-structured speckle pattern with bands containing mostly parallel fibrils while lower SFA has a less clear banded pattern and more disorganization.³⁴

In a case report published by Crawford et al in 2021, a patient had a bilateral debridement of the patellar tendon, and the head researcher evaluated the patellar tendon regularly utilizing SFA ultrasound methods.²⁸ In this case, they measured the Peak Spatial Frequency Radius (PSFR) of the patellar tendon at four different knee joint angles over the course of the 12-month rehabilitation period and assessed pain and dysfunction via the Victorian Institute of Sport-P (VISA-P). Over the course of the study, they observed an increase in PSFR throughout the rehabilitation protocol, indicating an increase in collagen organization. In another study, Kulig et al analyzed patellar tendon tissue for spatial frequency parameters in volleyball players and determined that symptomatic athletes demonstrated greater collagen disorganization compared to non-athlete controls and asymptomatic volleyball athletes.³⁵ These authors suggested that painful symptoms are associated with degeneration rather than inflammation only and that assessing patellar tendons with DU can help track changes in tendon structure to determine the proper course of treatment and rehabilitation.

Clinical Implications

Diagnostic ultrasound is cost-effective, non-invasive, reliable, and efficient as an imaging tool that can assist clinicians with the evaluation and diagnosing of conditions like patellar tendinopathy at the point of care. Utilizing DU can provide information related to the intrinsic characteristics of the patellar tendon that can more accurately describe the current state of an abnormal patellar tendon (Table 1). These can be leveraged to provide guidance to clinicians in terms of understanding the pathology, creating treatment and rehabilitation protocols for altered tendon morphology, and informing clinical decision making.

Point of care DU use in the orthopedic and sports medicine field is rapidly developing. There are clear benefits to implementing this imaging tool to inform practice and improve patient care. But the use of DU is limited depending on settings, accessibility, and user proficiency. Clinicians seeking to implement ultrasound into their daily routine require proper training in the physical utilization of this technology and the ability to interpret images to maximize the benefits of DU at the point of care.

Sarah L. Woelfel MA, LAT, ATC, is a first year, PhD student studying Biomechanics and Kinesiology at the University of Nebraska at Omaha.

Adam B. Rosen PhD, ATC, is an Associate Professor in the School of Health and Kinesiology, Director of the Athletic Training Program at the University of Nebraska at Omaha and co-director of the UNO Sports Medicine and Biomechanics Laboratory.