Anterior cruciate ligament (ACL) and thermography: a critical review of the scientific literature

Anterior cruciate ligament (ACL) and thermography: a critical review of the scientific literature

06/07/2021 By: Victor Escamilla & Alejandro del Estal Home

Anterior cruciate ligament injury has multiple physiological repercussions and thermography is a great tool to analyze them, in order to control acute and chronic processes, according to an extensive review of the scientific literature.

The anterior cruciate ligament injury is one of the most severe and with the highest incidence in sports practice, after those produced in the ankle joint. This is especially remarkable in sports where contact with an opponent, high neuromuscular load and movement mechanics require pivoting on the lower limb, changes of direction and landings repeatedly.

Epidemiology of anterior cruciate ligament injury in different sports.

In an epidemiological review of various sports disciplines in adolescents (Joseph et al., 2013), several very relevant data on this injury were observed:

  • The first one highlights the knee as the joint with the most surgeries. 50% of all knee interventions involve the ACL. Due to its lengthy rehabilitation process and surgical service costs, US insurers estimate the annual economic cost of ACL reconstruction at $ 1 billion.
  • The second data indicates that women are more likely to suffer this injury. They are 4 to 6 times more likely to suffer an ACL injury than men. In addition, it is an injury that relatively frequently leads to the end of the athlete’s sports career. At the same time, it can permanently affect daily physical activity, as this type of injury increases the risk of developing osteoarthritis prematurely and developing chronic pain.
  • The third and last piece of information is the relationship that the type of sport has with the injury. It is important to note that regardless of it, 74.9% of the injuries within the study occurred during competition. This may make us think that competition is a significant risk factor. The sports with the highest incidence for ACL injury in the group of women was soccer, followed by basketball. While, for men, American football and soccer were the sports with the highest incidence.

Thermography as a diagnosis support in anterior cruciate ligament injury

As we have seen, the ACL injury is one of the most worrying injuries in sports traumatology and, therefore, one of the most investigated by science. In a simple search in PubMed with the term “anterior cruciate ligament injury”, 16,247 results were obtained (image 1).

Search anterior cruciate ligament (ACL) injury
Image 1. Search in Pubmed for the term “anterior cruciate ligament injury”

However, it is curious that, if you just add the term “thermography”, you only get 5 results, of which the first (sorted by: best match) is in a study carried out with dogs (image 2).

Image 2. Search in Pubmed for the term “anterior cruciate ligament + thermography”

It is a multifactorial injury, where neuromuscular fatigue and tissue status stand out as the most important markers in the risk of triggering the injury

(Acevedo, R. J et al 2014).

In addition, it is an injury that, despite technological advances and evolution in training and pre-rehabilitation techniques, has an annual increase in its incidence. The Ekstrand research group (Waldén et al., 2016) point out that there is no reduction in the tendency of ACL injuries in professional teams that compete in international championships, as can be seen in Figure 1.

Evolution trend
Figure 1: Trend in ACL injury rate in male professional soccer players, depending on the season, from 2001 to 2005. Adapted from Waldén et al. (2016).

Besides, only two-thirds of the players who break the ACL continue to compete at the highest level three years after his recovery. Therefore, the rate of abandonment of high-performance sports practice is very high, even with all possible means for a successful recovery.

That is why the scientific development of new technologies such as thermography can represent a significant advance. The main advantage is that it helps to monitor the state of the athlete’s tissues during competitive periods, which facilitates the prevention of injuries. In addition, it helps to control the evolution of recovery after ACL surgery or conservative post-treatment. In this way, it is possible to analyze effusion processes during rehabilitation. These processes could slow down the return-to-play and are also a predisposing factor for the development of premature osteoarthritis. In this regard, thermography could be a key tool in monitoring these processes.

Scientific development of thermography for anterior cruciate ligament injury monitoring.

Anecdotally in 2010, Hildebrandt and collaborators cite this injury in a section of their article: “An overview of recent application of medical infrared thermography in sports medicine in Austria“. They show how thermography can be applied to evaluate traumatic injuries, taking the ACL as an example of injury in skiers (Hildebrandt et al., 2010).

Hildebrandt reported a single case of a skier who had injured his right knee 6 weeks ago (image 3, left). The image shows a hyperthermia around the patellar region, as a result of the inflammatory process, which was related to pain. In image 3 (right side), the same knee is represented 6 months after the extensive rehabilitation program, where a clear drop in temperature can be observed. Although a hyperthermic signal is still present, known as a “thermal scar”. It seems to stay above the structure for the long term, according to our experience, from some months to several years.

Image 3. Left: right knee with ACL injury of 6 weeks evolution. Right: same athlete, 6 months later. Adapted from Hildebrandt et al. (2010).

It was not until 2016, when the thermography group of the Faculty of Physical Activity and Sports Sciences, of the Polytechnic University of Madrid, published a thesis with the title: “Use of infrared thermography as a tool to monitor skin temperature along the recovery process of an anterior cruciate ligament surgery(Cano, 2016). In it, the ACL rehabilitation process is evaluated weekly for six weeks, complemented with a cross-sectional data collection at 18 months. The study sample consisted of 25 subjects, with a mean age of 32 years, most of whom were injured while playing sports. In the study, subjects followed a six-week rehabilitation program divided into three phases of progression.

In the first three weeks of rehabilitation, phase 1 was carried out, which consisted of range of motion restauration, pain modulation and the inflammation process. The aim was to restore the gait without external elements and to remove the orthopedic elements.

In the following three weeks, if the medical department authorized it, they advanced to phase 2. The objective was to obtain the full range of motion, adaptation to normal gait, increased strength and flexibility, cardiovascular conditioning, proprioception actions and cyclical activities such as bicycles. In addition, in the last week, reception and deceleration activities of the body were included after jumping on a mat.

From then on, they entered phase 3, where they were discharged so that they could return to their activities of daily living (ADL). At 18 months, they were contacted again for the cross-sectional evaluation.

The results show that the uninjured leg has a higher hyperthermic pattern than the injured leg throughout the posterior chain, which is statistically significant. This fact occurs both when rehabilitation begins and when it ends (Figure 2).

Figure 2: Maximum temperature values ​​for AV (Anterior Leg View) and PV (Posterior Leg View) for the injured and uninjured leg. Adapted from Cano (2016).

This can be mainly due to two factors:

  • The first one, the over-solicitation of the uninjured leg for ADLs. This could lead to compensating asymmetrically, by performing more actions and with a greater load on the healthy side. The author defines this as “functional overload due to compensation”. Due to the increase in the request of the soleus by involving its recruitment before contact with the ground (Cano, 2016). In addition to the anticipation of the deceleration of the body with the same stride length, which predominantly requires an increase in the activity of the hamstring complex (Colné & Thoumie, 2006).
  • The second factor, which is in line with the first, is that there is an inhibition of the posterior muscles of the injured leg. This occurs especially in the hamstring complex, due to the surgical technique, since the graft is removed using the semitendinosus tendon technique of the injured leg. This, together with the inhibition processes of the neuromuscular system, makes the temperature of the posterior chain of the injured leg decrease.

While, for the anterior chain, due to inflammation and biological tissue repair processes, there is a hyperthermic pattern in favor of the injured leg.

It should be noted that, if the data is investigated further, we find that the anterior chain is certainly hyperthermic, since there is a main source of heat in the knee that distorts the statistical analysis. A closer look at the ROI of the quadriceps of the injured leg reveals that there is a hypo therm with respect to the good leg (Figure 3).

Figure 3: Figure A, corresponds to the ROIs of the injured leg before and after finishing the rehabilitation for the values ​​of mean and maximum temperatures. Figure B, corresponds to the ROIs of the non-injured leg before and after finishing the rehabilitation for the values ​​of mean and maximum temperatures. Figure C, avatar with the ROIs selected in the study for the anterior and posterior view. Adapted from Cano (2016).

As with the inhibition of its antagonist, the quadriceps of the injured leg is also inhibited by the surgery. This is a phenomenon known as arthrogenic inhibition, very common in postoperative patients. It is curious that, although many efforts are devoted to strengthening and activating these muscles, the effects of cortical inhibition are maintained over time.

These effects are known as SICI (short interval intracortical inhibition) and are in line with research on the process that the quadriceps undergoes during rehabilitation of this injury

(Kuenze et al., 2015).

Finally, the authors showed findings 18 months later similar to those found in the study by Hildebrandt et al. (2010). Despite the fact that the elapsed time had been three times longer, there is a thermal footprint or scar, similar to that found at six months for injuries involving ligament tissue.

Therefore, it is proposed that thermal asymmetry should be assumed as an individualized profile of patients with ACL operation. It should be monitored to control the processes of premature osteoarthritis in cases in which the temperature of the joint varies from its new individual profile.

ThermoHuman case study: Olympic judoka with anterior cruciate ligament injury.

Next, we show you in image 4 the clinical case of an Olympic judoka. She was evaluated during her rehabilitation process for ACL reconstruction in her left knee. Arthrogenic inhibition of the quadriceps of the left leg can be observed, as discussed above. In addition, a greater stress on the contralateral leg is observed, probably a consequence of compensation in the ADLs. The sum of both factors results in hyperthermia in the left posterior chain (Image 4).

Judoka with anterior cruciate ligament rupture 2 months after the operation
Image 4: Judoka 2 months after the operation with clear signs of “functional compensation overload” and ROI of the injured knee with hyperthermic signal. Images property of ThermoHuman.

Furthermore, in the evolution if we analyze with the specific knee protocol, we can observe that the “thermal scar” is present in the left knee (Image 5).

Thermal scar in an anterior cruciate ligament patient
Image 5. Evolution with the knee-specific protocol to evaluate the “thermal scar”. Images property of ThermoHuman.

On the other hand, it is very interesting to note how the foot in its plantar view undergoes thermal changes (Image 6). In the initial stages of recovery we found a hyperthermic pattern in the foot of the uninjured leg, which could be related to a biomechanical compensation of the gait. Although this last phenomenon could also have a vascular explanation.

Specific analysis of ThermoHuman foot after two months of anterior cruciate ligament rehabilitation
Image 6. A) Specific analysis of the foot with ThermoHuman after two months of ACL rehabilitation, hyperthermia values in the non-injured leg. B) A) Specific analysis of the ThermoHuman foot after eight months of ACL rehabilitation, values close to symmetry.

Case Study: ACL Recovery

Tracking over 184 days of a professional athlete’s ACL injury during the recovery process in their knee ROI graph:

Since, Litscher in his research (Litscher et al., 2014) compared two types of manual therapies to patients with ACL reconstruction:

These authors found that classical therapy increased the local temperature of the knee, but no more adjacent areas. However, alternative therapy (RegentK) achieved a systemic response. This means a stimulation of the lymphatic system, improving oxygenation of the structure, and of the contralateral knee as a reflex cross effect orchestrated by the brain.

In addition, it was possible to increase the temperature of the foot (Figure 5). This could indicate that the inflammation over the knee reduces blood flow and information to the most distal part of the joint. Therefore, there is also a hypothermic asymmetry and not only as a consequence of biomechanical compensation in the injured foot (Litscher et al., 2013).

Changes in the temperature of the knee and foot in a anterior cruciate ligament patient
Figure 4. Figure A: changes in the maximum temperature of the knee and foot regions before and after the application of the alternative therapy (RegentK). Figure B: Changes in the thermograms before and after the application of the alternative therapy (RegentK). Adapted from Litscher et al. (2014).


Thermography appears to be a useful tool to monitor recovery processes after ACL reconstruction. In this critical review about the ACL injury seen with thermography, we consider there is low quality research on the progression of temperature throughout the recovery process.

The main conclusions drawn from the review of the literature and the follow-up of some clinical cases from ThermoHuman are:

  • The injury in the ligament tissue causes a hyperthermic consequence in the ROI of the injured knee. This hyperthermic signal generates an asymmetry with respect to the healthy knee that is prolonged in time.
  • Injured ACL knees have a time-fixed hyperthermic signal. In other words, ACL injuries generate a “thermal scar”. It must be integrated into the individual athlete’s profile to control more sensitive processes. For exemple, the premature development of knee osteoarthritis, as this asymmetry deviates from its normal pattern, as the metric of neutralized asymmetry show it.
  • Finally, during the recovery process, both the arthrogenic inhibition of the quadriceps and the hamstring complex are two fundamental objectives to be solved by the health and sports professional. Thermography makes it possible to monitor how these activation and compensation processes are on the part of the contralateral leg.


Acevedo, R. J. Rivera-Vega, A., Miranda, G., & Micheo, W. (2014). Anterior Cruciate Ligament Injury. Current Sports Medicine Reports, 13(3), 186–191.

Cano, S. (2016). Use of infrared thermography as a tool to monitor skin temperature along the recovery process of an anterior cruciate ligament surgery.

Colné, P., & Thoumie, P. (2006). Muscular compensation and lesion of the anterior cruciate ligament: Contribution of the soleus muscle during recovery from a forward fall. Clinical Biomechanics (Bristol, Avon), 21(8), 849-859.

Hildebrandt, C., Raschner, C., & Ammer, K. (2010). An Overview of Recent Application of Medical Infrared Thermography in Sports Medicine in Austria. Sensors (Basel, Switzerland), 10(5), 4700-4715.

Joseph, A. M., Collins, C. L., Henke, N. M., Yard, E. E., Fields, S. K., & Comstock, R. D. (2013). A multisport epidemiologic comparison of anterior cruciate ligament injuries in high school athletics. Journal of Athletic Training, 48(6), 810-817.

Kuenze, C. M., Hertel, J., Weltman, A., Diduch, D., Saliba, S. A., & Hart, J. M. (2015). Persistent neuromuscular and corticomotor quadriceps asymmetry after anterior cruciate ligament reconstruction. Journal of Athletic Training, 50(3), 303-312.

Litscher, G., Litscher, D., Ofner, M., Gaischek, I., & Malliga, D.-E. (2014). Temperature Measurements in Rehabilitation in Patients with Completely Ruptured Anterior Cruciate Ligament before and after RegentK and Physiotherapy. Medicines, 1(1), 12-21.

Litscher, G., Ofner, M., & Litscher, D. (2013). Manual Khalifa Therapy in Patients with Completely Ruptured Anterior Cruciate Ligament in the Knee: First Results from Near-Infrared Spectroscopy. North American Journal of Medical Sciences, 5(5), 320-324.

Viola, R. W., Steadman, J. R., Mair, S. D., Briggs, K. K., & Sterett, W. I. (1999). Anterior cruciate ligament injury incidence among male and female professional alpine skiers. The American Journal of Sports Medicine, 27(6), 792-795.

Waldén, M., Hägglund, M., Magnusson, H., & Ekstrand, J. (2016). ACL injuries in men’s professional football: A 15-year prospective study on time trends and return-to-play rates reveals only 65% of players still play at the top level 3 years after ACL rupture. British Journal of Sports Medicine, 50(12), 744-750.

Europa Thermohuman ThermoHuman has had the support of the Funds of the European Union and the Community of Madrid through the Operational Programme on Youth Employment. Likewise, ThermoHuman within the framework of the Export Initiation Program of ICEX NEXT, had the support of ICEX and the co-financing of the European Regional Development Fund (ERDF).

CDTI Thermohuman has received funding from the Centre for the Development of Industrial Technology (CDTI), in participation with the European Regional Development Fund (ERDF), for the R+D activities involved in creating a new tool, based on thermography, for the prediction and prevention of rheumatoid arthritis. See project detail.