According to our clinical observations, it is very likely to find MRI evidences of an ACL injury when patients refer with chief complaint of knee giving way and lateral knee pain. Thus, the presence of pain localized at the lateral aspect of knee usually hints us to consider ACL injury among other differential diagnoses. There are various causes for lateral knee pain which are mainly associated with an injury including iliotibial band syndrome, lateral ligament sprain, lateral cartilage meniscus injuries, PFPS, OA, biceps femoris tendinopathy or tendonitis, synovitis, and sprain of superior (proximal) tibiofibular joint. Although manifestation of lateral knee pain after ACL reconstruction has been addressed in several studies [12,26–29], the association between concurrence of lateral knee pain and ACL tear has not been noted in the literature yet. The data documented by VAS in the present study suggests that the severity of lateral knee pain significantly reduced in consecutive visits of pre- and post-operative periods. Also, the results obtained revealed that the percentage of the patients with severe pain decreased from 46.7% at the first preoperative visit to 3.3% at second postoperative visit; while as the percentage of patients with mild pain increased. These findings support the hypothesis of the correlation between lateral knee pain and ACL rupture. The number of studies investigating the lateral knee pain as their main concern is very limited. Kawaguchi et al.  noticed lateral knee pain in another aspect; They reported three cases complaining lateral knee pain during rehabilitation exercise post to their ACL reconstruction, and assumed that the lateral knee pain developed due to irritation between Tight Rope (RT) bottom and the autografts bundle used in ACL repair. The lateral knee pain in their cases only occurred when Kawaguchi et al. had surgically removed the RT bottom. Also, the case report of Anandkumar et al. (2018) described a 45-year-old Canadian female with lateral knee pain over the right proximal tibiofibular joint (PTFJ) managed unsuccessfully with rest, medications, bracing, injection, and physiotherapy. Finally, she was clinically diagnosed with PTFJ hypomobility . Also, they reported that the patient was discharged after four treatment sessions of Mulligan’s mobilization with movement and taping over the right PTFJ, and finally the patient was pain free and fully functional at the 6th month of her follow-up. However, the case reports of both Kawaguchi et al.  and Anandkumar et al.  do not assist us to provide any additional evidences to support the idea of the correlation between lateral knee pain and ACL injury, but a relatively rapid disappearance of lateral knee pain in response to a successful treatment poses the potential use of lateral knee pain in ACLR follow-up.
Additionally, the current study also investigated the probable influence of sex, age and type of harvested graft on lateral knee pain intensity. Regarding the type of autografts use, the difference of pain intensity in patients of both BPTB and HT groups was not significant but with a borderline p-value of 0.077. Since a limited number of patients were investigated in our study, the preference of the two methods of ACL reconstruction in terms of lateral knee pain requires more attention and in-depth studies. In contrast, Okoroha et al. (2016) reported that a significant increase in acute postoperative pain was found when ACL reconstruction was performed with BPTB compared to reconstruction with HS. This cohort study also discovered that patients treated with BPTB were more likely to contact their physician due to pain, and more likely to have a breakthrough pain and low satisfaction with their pain management . Moreover, in a prospective study with a 10-year follow-up, Pinczewski et al. (2007) reported that the number of patients with contralateral ACL re-rupture in the PT group (20) was significantly higher compared to that of HT group (9) (P = 0.02). Also, more patients reported pain with strenuous activities in PT knees than in HT knees (P = 0.05). Finally, these scientists concluded that excellent results could be obtained with both HT and PT autografts, but their final recommendation was in favor of 4-strand HT reconstruction to patients due to diminished harvest-site symptoms and radiographic osteoarthritis . Like Pinczewski’s study, a long term follow-up was not conducted in our pilot study. However, the relative precedence of PT graft over HT graft in Pinczewski’s study did not conform to that of our study.
The pain of other aspects of the knee is also addressed in the literature, particularly the association between anterior knee pain and ACL reconstruction. In this regard, Kovindha et al. (2019) noticed that both anterior knee pain and numbness improved over time (3 to 6 months post-operation) . Likewise, Pinczewski et al. (2007) reported that 33% of patients suffered from kneeling pain during a 2-year follow-up, and percentage of the patients experiencing kneeling pain during a 10-year follow-up increased to 59% . The causes of the discrepant results of these two studies with respect to the recovery time of anterior knee pain or incidence rate of kneeling pain are not our concern now, but a more general inference may negate the role of anterior knee pain and kneeling pain in the diagnosis of ACL tear, since these two often appear after ACLR. In addition, the need of a long-term follow-up for the evaluation of other knee pains except lateral knee pain makes us doubt about the assistance of anterior knee pain as well as kneeling pain in the assessment of the success of ACLR. Overall, the paucity of the literature regarding the correlation between lateral knee pain ACL rupture constrained us to find a sound basis for a comprehensive comparison between the studies. Nevertheless, the results of the present pilot may persuade the clinicians to consider the association between lateral knee pain and ACLR as well as efficacy of its operative treatment.
The limitations of the current study include a small sample size, discrepancies in rehabilitation and restorative cares maintained by patients, and the difference in rehabilitation equipment used by our patients. Therefore, conduction of further studies with a larger sample size, a randomized sampling from several hospitals, equal numbers of participants from both genders, and similar surgical method with respect to fixation and graft is recommended. Also, evaluation of the etiology of lateral knee pain in the general population can give a better understanding of the specificity of this symptom in diagnosing ACL injuries.