Prevalence of patella baja in primary total knee arthroplasty

INTRODUCTION

Total knee arthroplasty (TKA) is considered one of the most reliable and successful surgical interventions, with consistently favorable outcomes.^[1] TKA involves reconstructing the knee joint by replacing its articular surfaces with a combination of smooth metal (cobalt-chrome or titanium) and highly cross-linked tibial polyethylene.^[2] The patella is embedded deeply between the two tendons, linking the quadriceps and patellar tendons, allowing it to displace the extensor mechanism anteriorly, thereby increasing the mechanical efficiency of the quadriceps muscle.^[3] However, any alteration in patellar height can disrupt this critical biomechanical bond between the tendons. Decreased patellar height, known as patella baja (PB), and increased height, referred to as patella alta, can both lead to abnormal joint reactive forces.^[3]

PB (also known as patella infera) is a condition that may be congenital, acquired (due to injury or surgical intervention), or a combination of both.^[4] It is a frequently encountered finding and/or complication following TKA.^[5] PB is characterized by an abnormally low-positioned patella that remains distal to the femoral trochlea, causing the patella to stay engaged in the trochlea throughout the entire range of motion (ROM) rather than disengaging from the trochlea during terminal extension.^[6] This altered movement results in anterior knee pain, extensor lag, and reduced ROM due to a shortened lever arm.^[3] Furthermore, the presence of PB increases the risk of patellar tendon rupture or avulsion in patients undergoing TKA.^[7]

Patella height can be determined using the Insall–Salvati ratio (ISR) or Blackburne–Peel ratio (BPR) on a 30° flexed lateral knee radiograph. The ISR was calculated by measuring the patellar tendon and patellar length ratios. A normal ISR falls between 0.8 and 1.2, with values <0.8 indicating PB and patella alta if the ratio is above 1.2.^[8] Conversely, the BPR is determined by two vertical measurements from a horizontal line drawn at the level of the tibial plateau. These two measurements are the patellar articular surface and the distance between the horizontal line and the inferior aspect of the patellar articular surface.^[9] A normal BPR ranges from 0.8 to 1.0, with ratios below 0.8 indicating PB and those above 1.0 indicating patella alta.^[9,10]

This study was necessitated by the need to address the knowledge gap regarding the prevalence of PB in the Saudi population who underwent TKA. While TKA is a widely performed procedure with established success rates, anatomical variations, such as PB, can significantly impact surgical outcomes and patient recovery.

The current literature predominantly reports on PB prevalence in Western and Asian populations, which may not accurately reflect the anatomical and demographic characteristics of patients from the Middle Eastern population. By focusing on the Saudi population cohort, this study aimed to provide critical insights into the incidence of PB, which may influence surgical techniques and postoperative management in this population.

Understanding the prevalence and implications of PB in our patients will enhance orthopedic surgeons’ clinical awareness and facilitate the development of tailored surgical approaches for its management. This study is crucial for improving patient outcomes and reducing complications associated with TKA, particularly given the unique anatomical considerations of the region.

MATERIALS AND METHODS

Measurements

The ISR and BPR were both used to assess the patellar position, but they focus on different anatomical landmarks and measurements, which can yield different results in diagnosing conditions such as PB [Figure 1].

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Figure 1:

An illustration of the measurements taken for the Insall– Salvati ratio and for the Blackburne–Peel ratio. Q: Drawn tangent to the tibial plateau articular surface in the sagittal plane; R: The patellar tendon length; S : The patellar length, from superior pole to inferior; T: Length of the patellar articular surface; V: Line drawn perpendicularly from a horizontal reference line across the tibial plateau to the inferior articular surface of the patella.

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RESULTS

This study included 106 knees from patients who underwent primary TKA. Among these, 85 knees (80.19%) were from female patients and 21 (19.81%) were from male patients. The average age of the patients was 64.91 (±11.44) years. Regarding the knee distribution, 54 (50.94%) and 52 (49.06%) knees were on the right and left sides, respectively [Table 1].

Various key measurements were obtained for the patella, including the means of the LP of 40.37 (±4.60) mm, the LT of 37.43 (±6.19) mm, the tendon height of 23.78 (±6.61) mm, and the patellar height of 31.23 (±3.23) mm. The calculated mean ISR was 0.938 (±0.18), while the mean BPR was 0.767 (±0.22) [Table 1].

Based on the ISR index,^[10] 14 knees (13.21%) were classified as PB, whereas 59 knees (55.66%) fell within the normal range (0.75–0.99). In addition, 33 knees (31.13%) were categorized as having patellar alta. Regarding the BPR index,^[10] 69 knees (65.09%) were classified as PB, 26 knees (24.53%) fell within the normal range (0.80–1.0), and 11 knees (10.38%) were categorized as patella alta [Table 1].

As shown in Figure 2, the ISR showed no significant difference between males (median = 0.939, interquartile range [IQR] = 0.246) and females (median = 0.934, IQR = 0.224; P = 0.32). Similarly, the BPR showed no significant difference between men (median = 0.719, IQR = 0.251) and women (median = 0.776, IQR = 0.238), with a P = 0.32 [Figure 3]. Moreover, the difference between the two methods for measuring PB was statistically significant (χ²1) = 49.59, P < 0.001 [Table 2].

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Figure 2:

Mann–Whitney U comparison between Insall–Salvati ratio and sex.

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Figure 3:

Mann–Whitney U comparison between Blackburne–Peel ratio and sex. ns: Not significance

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DISCUSSION

TKA typically involves medial parapatellar arthrotomy, followed by patellar eversion or subluxation.^[12] The occurrence of PB during TKA can be attributed to congenital, acquired, or a combination of both factors.^[1] PB presents two main technical challenges, as patients often undergo multiple prior surgical procedures: The risk of patellar tendon avulsion and joint line elevation, particularly when distal femoral resection is performed to optimize the extension gap.^[4] These technical challenges may contribute to prolonged surgical duration and an increased risk for complications.

First, patellar tendon avulsion is a significant injury resulting from excessive tension applied to the patellar ligament or quadriceps tendon, leading to the detachment of the patellar tendons from the tibial tuberosity.^[13] This condition can complicate patellar tendon eversion, a procedure typically performed to enhance the exposure of the lateral tibial plateau.^[14]

Second, there is an increased risk of joint line elevation if excessive bone is resected from the distal femur to optimize the extension space.^[15] Joint line elevation can lead to pseudo-PB, a complication that surgeons should actively avoid.^[15] To mitigate this, the inferior patellar bone should remain in a resting position; thus, resection of the lower portion of the patella may create space to prevent tibial impingement. A revision femoral component with distal augmentation may be used in more severe cases that require lowering the joint line (distalization).^[16] A serious concern arises in pre-existing true PB cases, which may cause severe biomechanical effects.^[17]

Specific populations, particularly Asians, are known to have smaller knee sizes, often accompanied by genu varum and external tibial torsion.^[18] Thus, surgeons must be cautious and aware of the potential complications associated with overlooked PB. To mitigate the risk of patellar tendon avulsion in these patients, several preventive strategies can be employed, including avoiding patellar eversion, opting for patellar subluxation, and applying a protective smooth pin in the patellar tendon near its attachment to the tibial tuberosity.^[19] Furthermore, a rectus snip can be performed to obtain optimal visualization of the knee when the PB severely limits the surgeon’s view.^[20]

Patella alta and PB affect 1–2% of the global population and are underreported, which may lead to complications such as osteoarthritis.^[21] Studies have reported that patellar morphology can vary across populations due to habitual behaviors, such as frequent kneeling, squatting, and cross-legged sitting.^[21] In 2019, the Mayo Clinic conducted a multicenter retrospective study to determine the incidence of PB before and after primary TKA, stratified by patients’ BMI.^[22] The authors reported that the prevalence of PB among patients with normal BMI was 6% and 5%.^[22] A 2020 study from South Korea indicated that 19.8% of the population had PB, as measured using the ISR index.^[23]

Our analysis focused on 106 knees from patients who underwent TKA, specifically examining the prevalence and measurement of PB. The PB classification was based on two established guidelines: The BPR and ISR. According to the BPR, 65.09% of knees were classified as having PB, whereas the ISR indicated a significantly lower proportion (13.21%). These findings suggest a notable variation in PB classification depending on the measurement guidelines used, with our results showing a higher incidence than that in the Western population and a lower incidence than that in the Asian population.

The difference between the two ratios may stem from measurement, anatomical, pathological, and methodological factors. The ISR focuses on the size and length of the tendon and patella, whereas the BPR considers the relationship between the patella and the tibial plateau. Consequently, variations in these measurements can lead to different results. Individual anatomical differences, such as variations in tendon length or patellar height, and pathological changes, such as tendon degeneration, may affect one ratio more than the other. However, in this study, we minimized discrepancies related to methodological factors by standardizing the measurement method. Therefore, conflicting interpretations of patellar positioning are possible when using the ISR and BPR. Clinicians often consider both ratios along with clinical findings to make a comprehensive assessment of the patient.

No sex differences were observed in ISR or BPR (P = 0.32 for both [Figures 2 and 3]), indicating that PB classification is independent of sex in this cohort, which is a key insight for equitable surgical planning. This study also highlighted the prevalence of PB in the study population, with notable differences between the ISR and BPR methods used. The ISR classified a lower percentage of knees as PB than the BPR, suggesting that the choice of measurement method can yield different clinical interpretations of PB. This finding underscores the importance of clinicians’ awareness of potential discrepancies between these two ratios when making assessments and patient management decisions. Furthermore, this study underscores the importance of standardizing measurement techniques to minimize methodological discrepancies that may affect clinical outcomes. Because individual anatomical and pathological factors can influence outcomes, this study advocates a comprehensive assessment that incorporates both ratios and clinical findings to ensure accurate evaluation of patellar positioning. Our high BPR-based prevalence suggests that routine preoperative screening in Saudi cohorts could mitigate these issues, although prospective data are needed. Overall, the findings have significant implications for clinical practice, as they encourage a more nuanced approach to assessing patellar positioning in patients undergoing TKA, ultimately aiming to improve surgical outcomes and patient care. In PB cases, surgeons should prioritize techniques such as patellar subluxation over eversion to avoid avulsion (risk reduced by 50%^[19]) and monitor the joint line to prevent pseudo-PB.^[15] Our high BPR-based prevalence suggests that routine preoperative screening in Saudi cohorts could mitigate these issues, although prospective data are needed.

CONCLUSION

This study found a notably higher prevalence of PB among patients undergoing primary TKA in our cohort than that reported in the existing literature. This anatomical variation poses significant technical challenges during surgery and, if not addressed properly, can lead to complications that adversely affect patient outcomes. Given the elevated incidence of PB, it is imperative that surgeons remain vigilant and account for this variation when planning and performing TKA.

Adopting appropriate surgical techniques tailored to manage PB effectively is crucial for optimizing surgical outcomes and minimizing associated risks. Future research should ensure that orthopedic practices evolve to meet the needs of all patient demographics.