Excessive Apoptosis in Patellar Tendinopathy in Athletes

ystein Lian, MD^1*, Alex Scott², Lars Engebretsen, MD, PhD³, Roald Bahr, MD, PhD⁴, Vincent Duronio², Karim Khan, MD, PhD²

¹ Oslo Sport Trauma Research Center, Department of Sports Medicine, Norwegian School of Sport Sciences, Oslo, Norway; Kristiansund Hospital, Kristiansund, Norway
² University of British Columbia, Vancouver, Canada
³ Oslo Sport Trauma Research Center, Department of Sports Medicine, Norwegian School of Sport Sciences, Oslo, Norway; Orthopaedic Center, Ullevål University Hospital, Oslo, Norway
⁴ Oslo Sport Trauma Research Center, Department of Sports Medicine, Norwegian School of Sport Sciences, Oslo, Norway

^* To whom correspondence should be addressed. E-mail: ostrc{at}nih.no.

	Abstract

Background: The pathogenesis of tendon overuse injuries is poorly understood. The histopathology underlying tendinopathy at various anatomical locations is similar and may reflect a common pathologic process.

Hypothesis: Apoptosis contributes to the pathophysiology in patellar tendinopathy.

Study Design: Case control study; Level of evidence, 3.

Methods: We compared biopsy specimens from the patellar tendon in patients with patellar tendinopathy diagnosed clinically and with typical magnetic resonance image findings with biopsy specimens from a control group without any previous or current knee complaints to suggest patellar tendinopathy. The presence of apoptosis was examined with immunohistochemical methods using a polyclonal antibody recognizing active caspase-3, confirmed by labeling DNA strand breaks (F7-26 antibody) and nuclear morphology (fragmentation and condensation).

Results: The number of apoptotic cells per unit area (4.5 mm²) was 0.91 ± 0.81 (SD) in tendinopathic samples and 0.21 ± 0.21 in controls (P = .026). Although the tendinopathic samples displayed increased cellularity (average 162.5 nuclei/mm² vs 98.9 nuclei/mm²), the apoptotic index was higher (0.42% vs 0.17%, P = .014).

Conclusion: Increased apoptotic cell death is a feature of patellar tendinosis. The role of apoptosis within the broader framework and time course of tendon overuse injury remains to be established.

Key Words: tendon, patellar tendinosis, apoptosis

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