1 Introduction

The knee joint is prone to both natural degeneration and trauma. The main reason for this is its anatomical location that makes it susceptible to injuries, its range of motion and its biomechanics which involve different loads passing from the joint in relation to different angles.^1,2 Movement and stability are achieved by a combination of bony and soft tissue components with biomechanical significance such as the Anterior Cruciate ligament (ACL) that offers stability and the patella that acts as a lever for motion enhancement.^2,3 Anatomical variations of these elements can result in joint pathologies.¹ Structural abnormalities of the patella have been associated with different disorders of the knee. Abnormal patellar height refers to patella Alta and Baja, meaning a superior and an inferior patella respectively.⁴ In patella Alta there is malalignment and decreased contact between the patella and the femur; as a result patellofemoral contact pressure increases.^5,6 Similarly, in patella Baja there are decreased restraining forces laterally causing a decreased range of motion, tibial tubercle osteochondrosis and early patellofemoral arthritis.^5,6 Abnormal position of the patella was found to be a significant contributing factor to ACL tears, patellar cartilage lesions, patellar tendinopathy, recurrent patellar dislocation/sublaxations and instability; however, there is no published research to our knowledge that has investigated its effect on meniscal tears.^2,7–13

Injury to the meniscus is a common clinical entity that troubles thousands of patients annually while its diagnosis requires clinical examination, expensive imaging and often arthroscopic interventions. The standard method for determining meniscal pathologies remains physical examination after obtaining a detailed history. This involves examining the knee for tenderness over the joint, extension lag, pain and clicking on passive manipulations, as well as various tests to assist diagnosis with variable degrees of sensitivity and specificity.¹⁴ Finally, the co-existence of an ACL pathology should always be investigated.¹⁴ Although valuable in diagnosis clinical examination is often followed by MRI scans and arthroscopy to better visualize lesions.¹⁴ In the past it has been shown that injury to the medial meniscus was more frequent in patients with patella chondromalacia, which was attributed to the resulting poor joint biomechanics and degeneration.¹ In this research article we are investigating whether the height of patella is related to meniscal tears. We aim to see if an abnormally high or low patella (patella Alta or Baja) predisposes patients to meniscal tears. In a population of 100 patients with meniscal tears the percentage of those with an abnormal patellar height is identified and compared to the percentage of those with a normally positioned patella. If the abnormal patellar height as an anatomical variation is proven significant as a predisposing factor to meniscal tears it can be implemented as a prognostic factor or index for identification of lesion-susceptible cases. If disproven it will show that the patellar height although relevant to other injuries such as patellar cartilage damage and ACL lesions is not relevant to meniscal tears.^3,7,12,15 The clinical significance of identifying factors related to such lesions lies not only in the early recognition of patients prone to injure their menisci in the community but also in the sports industry where these injuries and their rehabilitation cause substantial delays and are particularly costly. The identification of athletes prone to damaging their menisci due to anatomical variations can result in early prevention with the use of protective equipment in training and performing, as well as personalized strengthening exercises. The exact mechanism by which the patella position/malposition affects the meniscus is unclear and is related to the biomechanics of the joint.

2 Methods

100 patients, 80 males and 20 females who presented to our outpatient clinic due to knee pain after trauma from February 2018 to March 2019 represent the study population.

Only adult patients were included in the study, between the ages of 18 and 56 years. Excluded were patients who presented with co-existing lesions such as anterior cruciate ligament or medial collateral ligament tears; patients with associated fractures and those with severe knee osteoarthritis.

Moreover, patients with limb-length discrepancies, varus and valgus deformity of the knees were excluded.

X-rays were obtained in an anteroposterior and a lateral view, the latter with the knee in 30° flexion. Many methods have been suggested for measuring the patellar height, including the Insall-Salvati (IS) ratio, the Modified IS, the Caton-Deschamps (CD) index and the knee triangular ratio.⁶ Insall-Ssalvati and Caton-Deschamps were used in this study as they have both been reported reliable in several research papers. Reference ranges for the measurements are shown in Table 1.^8,16

Measurements were done using the Image-J program on the lateral X-rays in 30° knee flexion as this is the view that clearly shows the patellar tendon and its insertion into the tibia.^17,18 As such, the ratio between the patellar tendon length to the vertical patellar length gave the Insall-Salvati, while the distance between the articular facet of patella and the anterior corner of the superior tibial epiphysis to the patellar articular facet gave the Catton-Deschamps index (Fig. 1).^9,19 MRI was used to carefully examine and classify the meniscal lesions and to exclude any ligamentous pathologies. Finally, knee arthroscopy was performed in order to properly investigate the joint, confirm the clinical and radiographic/MRI findings and to repair lesions.

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Fig. 1 Insall-Salvati ratio indicated by yellow: Patellar Tendon Length (PTL) to Patellar Length (PL); Caton-Deschamps index indicated by green: Patello-Tibial Distance (PTD) to Patellar Articular Facet Length (PAFL).

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X-ray and MRI evaluation and ImageJ measurements were done by a Consultant Radiologist and two Orthopaedic Consultants. Results were statistically analysed on Microsoft Excel.

3 Results

The study included 100 patients with isolated meniscal tears. 4 of them were diagnosed with patella Baja according to both the IS ratio and the CD index, corresponding to 4% of the research population. 16 were found to have patella Alta based on the IS index, whereas only 5 according to the CD index measuring 16% and 5% respectively. Noted is the great deviation in the case of patella Alta measurement between the two ratios. Normal patella heights accounted for 80 patients base on IS and 91 based on CD. Measurements are summarised in Table 2.

Patella Alta occurred 3 times more often in males according to IS ratio and 2 times more often according to CD ratio, whereas patella Baja was seen twice more frequently in females based on both ratios. Overall it is evident that the two ratios agree on the measurement of patella Baja but are not concordant in the case of patella Alta.

According to the Insall-Salvati ratio 20% of the study population had an abnormal patella, while based on Caton-Deschamps index an abnormal patella height was found in 9% of the population with isolated meniscal tears. A statistical t-Test analysis of the results yield p = 0.8 for the Caton-Deschamps index, suggesting no statistical significance against the p value of 0.05 and p = 0.01 for the Insall-Salvati ratio, indicating significant results.

The most common lesion in concordance to other studies was a medial one, 75%, whereas lateral lesions were seen in 20% and only in 5% of patients both menisci were torn.¹ One interesting observation was the relevance of MRI findings to the ones of the arthroscopy; specifically, in 20 out of 100 patients MRI results were incorrect or less informative than the arthroscopy.

4 Discussion

The patella is a key structure to knee biomechanics.^1,3 Its position if abnormal (Alta/Baja) has been shown to result in ACL tears, patellar cartilage lesions, patellar tendinopathy, recurrent patellar dislocation/sublaxations and instability; however, its effect on the integrity of the meniscus is unknown. This research is a first investigation of the relation of the patellar height to meniscal injuries.^2,7–13

The normal patellar height can be derived from the ratio between the patellar tendon length and the vertical patellar height with a 20% acceptable variation based on anatomic differences.^9,18 Patella height variations, defined as patella Alta or Baja can be determined among others by the Insall-Salvati (IS) ratio, the modified IS ratio, the Caton-Deschamps Index, the Blackburne-Peel and the Knee triangular ratio.^5,18 According to Insall-Salvati the patella is referred to as Baja if the ratio is < 0.8, normal if it is between 0.8 and 1.2 and Alta in >1.2 (Table 1).¹⁸ The Caton-Deschamps Index defines patella as Baja in <0.6, normal between 0.6 and 1.3 and Alta in >1.3 (Table 1).¹⁹

Patella height was measured in 100 patients with isolated meniscal injuries in order to determine the percentage of cases in which this was abnormal. Measurements done according to the IS ratio resulted in 20% abnormal patella positions and those done with the CD index resulted in 9% abnormal results. In this study we are not aiming to investigate the superiority of one of the two measurements, IS or CD. The difference between the two is great, nevertheless both have been reported as reliable in previous studies and their comparison is beyond our scopes.^6,8,15,18,19 Lateral radiographic studies of the knee in 30° and 90° flexion demonstrated dissimilarities between IS and CD indexes on the same joint without suggesting a clear aetiology as to why such a difference between measurements exists.^20,21 A suspicion is that anatomical variations could disrupt measurements, affect indexes and alter results, such as morphological variations of the patella (i.e. long-nosed) can influence the reference points used to determine the Insall-Salvati ratio and a patella Alta can be misdiagnosed as normal.²² Similarly, the Caton-Deschamps index could be influenced by an anatomical variation of the patellar articular surface.²² It is evident that both measurements of 20% and 9% suggest the existence of an interesting relationship between the position of the patella and the meniscal integrity. In our 100 patient sample the abnormal position of the patella was found statistically significant as measured by the IS ratio (p = 0.01) as opposed to the CD index measurements which did not show significance (p = 0.8). A further step to investigate for correlation would be to extend this research to greater populations and even to try and determine the amount of patients from a group with abnormal patella height who develop meniscal tears. The mechanism behind this suggested relationship is still unclear. Insight could be given by biomechanical studies of the patella-meniscus dynamics. This research is a first investigation of the existence of such a relation and our results are promising for the existence of a connection.

5 Conclusion

20% of our 100-patient research group with meniscal tears had an abnormally positioned patella according to the Insall-Salvati ratio and 9% according to the Caton-Deschamps index. Although statistical significance between the abnormal height of patella and meniscal lesions was shown by the IS ratio measurements, the small sample size calls for further exploration. This research represents a first investigation into this relationship and shows promising results of a correlation. More attention should be paid to this interesting anatomic parameter and more studies are necessary to further investigate its relation to meniscal injuries in greater populations.