In this article, we will explore the impact of anterior cruciate ligament (ACL) injuries in professional football, highlighting the importance of effective monitoring during rehabilitation. We will focus on how infrared thermography (IRT) emerges as an innovative tool to assess knee health and guide the return-to-play process.

Furthermore, we will analyze a study that evaluates the use of thermography in football players during their recovery after ACL surgery. We will also identify thermal patterns that can help optimize return-to-play protocols.

Anterior cruciate ligament (ACL) injuries are one of the main causes of absence in professional soccer, with an incidence of 1.2 injuries per 1,000 hours of exposure (López-Valenciano et al., 2020). These injuries have a significant impact both in sports and economic terms, with estimated costs ranging between €84,000 and €499,000 per player in major European leagues (Pulici et al., 2023). Only 65% of players manage to regain their peak competitive level after ACL reconstruction, highlighting the need for effective rehabilitation protocols.

In this context, infrared thermography (IRT) presents itself as an innovative, objective, and non-invasive tool that allows for monitoring the rehabilitation process and assessing knee health. Its ability to measure skin temperature (Tsk) and detect thermal asymmetries provides essential data to guide return-to-play (RTP) decisions.

Thermography in Return-to-Play (RTP)

Thermography has been widely studied in clinical and sports contexts due to its ability to measure the physiological response of tissues (Fernández-Cuevas et al., 2015). In ACL rehabilitation, this technology helps identify residual inflammation and monitor tissue evolution throughout the recovery process.

Recent studies have shown that thermal asymmetries between knees act as a reliable indicator of physiological status, enabling medical staff to establish objective patterns of progress (Vardasca et al., 2012). This information is particularly useful to complement traditional functional and psychological assessments.

This study by Escamilla-Galindo et al. (2024) describes the use of thermography in soccer players during their rehabilitation following ACL surgery. It proposes objective thermal evolution patterns to complement traditional criteria and improve decision-making for return-to-play.

Methodology

The study included 30 professional and semi-professional soccer players (23 men and 7 women) from three European leagues, all undergoing ACL reconstructive surgery. Over one year, thermographic evaluations were conducted at nine key moments, from the first 48 hours post-surgery to 365 days afterward.

The protocol developed in this study included:

• Periodic evaluations: Measurements conducted on specific rehabilitation days (2, 9, 20, 35, 63, 126, 187, 274, and 365 days post-surgery).
• Controlled conditions: Assessments carried out in neutral environments following recommendations from the Thermographic Imaging in Sports and Exercise Medicine (TISEM) consensus.
• Specialized software: Use of ThermoHuman software, which facilitates automatic segmentation of regions of interest (ROI) and analysis of thermal asymmetries..

Results

The study identified a significant reduction in knee thermal asymmetries during the rehabilitation process for anterior cruciate ligament (ACL) injuries. Initially, the average asymmetry between the operated and healthy knee was 2.52°C in the early postoperative days. Over the one-year follow-up, this difference progressively decreased, reaching an average of 0.46°C by day 365, demonstrating consistent improvement in the physiological status of the affected tissue.

Evolution of Thermal Asymmetries

• Initial phase (Days 2–35):
  • During this period, the greatest thermal variability was observed, reflecting acute inflammation after surgery. This stage corresponds to the initial tissue recovery, reduction of edema, and gradual restoration of gait.
  • Early evaluations (Days 2 and 9) showed considerable dispersion, indicative of individual differences in the response to initial treatment.
• Intermediate phase (Days 35–187):
  • Thermal asymmetries began to stabilize, reflecting a transition toward functional rehabilitation. This period coincides with the introduction of progressive loading, functional exercises, and the removal of orthopedic aids.
  • Some players exhibited slight temporary increases in asymmetries, possibly related to tissue adaptation to new functional demands, a phenomenon interpreted as an expected physiological reaction.
• Advanced phase (Days 187–365):
  • From Day 187, a more pronounced decrease in thermal asymmetries was observed, parallel to graft maturation and adaptation to sport-specific training loads.
  • In the final months of rehabilitation, thermal asymmetries stabilized at low levels, though with a residual "thermal scar" averaging +0.46°C at the end of the period. This persistent hyperthermic profile suggests the need for long-term monitoring to prevent potential complications such as osteoarthritis.
    

This graph shows the evolution of thermal asymmetries over the nine evaluation moments:

• Early evaluations (Days 2 and 9): High value dispersion reflects acute inflammation and individual differences in immediate postoperative status.
• Intermediate stabilization (Days 35–187): Dispersion decreases over time, indicating a more homogeneous response among participants and more consistent thermal asymmetry values.
• Final stage (Days 274 and 365): Values become further compressed, stabilizing at levels that do not equal those of a healthy knee. This supports the idea of a residual thermal scar with potential long-term implications for joint functionality and health..

This pattern of progressive reduction (-0.01°C/day) highlights the value of thermography as an objective tool for monitoring the rehabilitation process. It establishes clear milestones for evaluating recovery and determining the optimal time for returning to sport. The final thermal difference of +0.46°C confirms the persistence of physiological alterations in the operated knee, which could have long-term implications for functionality and the risk of osteoarthritis.

Conclusions

• Reduction of thermal asymmetry: The operated knee's temperature progressively decreases by -0.01°C per day, reaching an average asymmetry of 0.46°C after one year.
  
• Persistence of a hyperthermic profile: At the end of the process, the operated knee maintains a residual thermal asymmetry of +0.46°C, consistent with previous research on "thermal scars" (Piñonosa et al., 2013).
  
• Infrared thermography is positioned as a key tool for monitoring and ensuring a safe return to sport, providing objective criteria that complement other assessments.

References

• Escamilla-Galindo, V. L., Felipe, J. L., Alonso-Callejo, A., Van-der-Horst, R., de la Torre-Combarros, A., Minafra, P., ... & Fernández-Cuevas, I. (2024). Return-to-play criteria based on infrared thermography during anterior cruciate ligament rehabilitation in football players. Biology of Sport, 42(2), 161-167
• López-Valenciano A, Ruiz-Pérez I, Garcia-Gómez A, et al. Epidemiology of injuries in professional football: a systematic review and meta-analysis. Br J Sports Med/ 2020; 54(12):711–718.
• Pulici L, Certa D, Zago M, et al. Injury burden in professional european football (soccer): Systematic review, meta- analysis, and economic considerations. Clin J Sport Med. 2023; 33(4):450–457.
• Vardasca R, Ring E, Plassmann P, et al. Thermal symmetry of the upper and lower extremities in healthy subjects. Thermol Int. 2012; 22(2):53–60.
• Fernández-Cuevas I, Marins JCB, Lastras JA, et al. Classification of factors influencing the use of infrared thermography in humans: A review. Infrared Phys Techn. 2015; 71:28–55