Abstract. Introduction. There are several imaging-based measurements for patello-femoral height. Available methods rely predominantly on sagittal images. The latter can be misleading with sagittal oblique slices and when the patella is tilted and/or chronically subluxed. In this study we describe a simple method of patellar height measurement using axial MRI overlap. Materials and methods. A retrospective observational analysis of 97 knees from 251 patients was conducted. Cases were selected following the exclusion of scans with fractures, massive effusion, patello-femoral pathology. Axial patello-trochlear overlap (APTO) was measured on the axial MRI images as follows: (1) Patellar length (P): expressed as the number of axial images showing patellar articular surface (2) Trochlear overlap (T): the number of axial images showing overlap between patellar articular surface and articular surface of lateral trochlea. APTO is the ratio T/P. All measurements were carried out independently and on two separate occasions by 6 raters. As a control conventional patello-trochlear index were measured for all patients by a senior musculoskeletal radiologist. Results. The mean APTO value was 36.7 (range 14.2 to 66.6; SD 11.4). There was a positive correlation with patello-trochlear index (Pearson correlation coefficient: 0.76, P < 0.001). Intra-observer reliability was good (ICC: 0.66 95 CI 0.54, 0.76, P < 0.001). Inter-observer reliability was fair (ICC: 0.51, 95 CI 0.41, 0.6, P < 0.001). Conclusion. In the present proof of concept study APTO was a reliable measurement of patellar height and correlated with patella-trochlear indices. The method described can prove valuable in overcoming issues with sagittal image measurements

Aims. This study aimed to investigate the relationship between changes in patellar height and clinical outcomes at a mean follow-up of 7.7 years (5 to 10) after fixed-bearing posterior-stabilized total knee arthroplasty (PS-TKA). Methods. We retrospectively evaluated knee radiographs of 165 knees, which underwent fixed-bearing PS-TKA with patella resurfacing. The incidence of patella baja and changes in patellar height over a minimum of five years of follow-up were determined using Insall-Salvati ratio (ISR) measurement. We examined whether patella baja (ISR < 0.8) at final follow-up affected clinical outcomes, knee joint range of motion (ROM), and Knee Society Score (KSS). We also assessed inter- and intrarater reliability of ISR measurements and focused on the relationship between patellar height reduction beyond measurement error and clinical outcomes. Results. The ISR gradually decreased over five years after TKA, and finally 33 patients (20.0%) had patella baja. Patella baja at the final follow-up was not related to passive knee ROM or KSS. Interestingly, when we divided into two groups - patella baja and patella normal-alta (ISR ≥ 0.8) - the patella baja group already had a lower patellar height before surgery, compared with the patella normal-alta group. The ISR measurement error in this study was 0.17. Both passive knee flexion and KSS were significantly decreased in the group with a decrease in ISR of ≥ 0.17 at final follow-up. Conclusion. Patellar height gradually decreased over five years of follow-up after TKA. The reduction in patellar height beyond measurement error following TKA was associated with lower clinical outcomes. Cite this article: Bone Jt Open 2021;2(12):1075–1081

Patellar instability most frequently presents during adolescence. Congenital and infantile dislocation of the patella is a distinct entity from adolescent instability and measurable abnormalities may be present at birth. In the normal patellofemoral joint an increase in quadriceps angle and patellar height are matched by an increase in trochlear depth as the joint matures. Adolescent instability may herald a lifelong condition leading to chronic disability and arthritis.

Restoring normal anatomy by trochleoplasty, tibial tubercle transfer or medial patellofemoral ligament (MPFL) reconstruction in the young adult prevents further instability. Although these techniques are proven in the young adult, they may cause growth arrest and deformity where the physis is open. A vigorous non-operative strategy may permit delay of surgery until growth is complete. Where non-operative treatment has failed a modified MPFL reconstruction may be performed to maintain stability until physeal closure permits anatomical reconstruction. If significant growth remains an extraosseous reconstruction of the MPFL may impart the lowest risk to the physis. If minor growth remains image intensifier guided placement of femoral intraosseous fixation may impart a small, but acceptable, risk to the physis.

This paper presents and discusses the literature relating to adolescent instability and provides a framework for management of these patients.

Cite this article: Bone Joint J 2017;99-B:159–70.

Background. Patellar instability is a complex, multi-factorial disorder. Radiological assessment is regarded as an important part of the management of this population. The purpose of this study was to determine the intra- and inter-rater reliability of common radiological measurements used to evaluate patellar instability. Methods. One hundred and fifty x-rays from 51 individuals were reviewed by five reviewers: two orthopaedic trainees, a radiological trainee, a consultant radiologist and an orthopaedic physiotherapist. Radiological measurements assessed included patellar shape, sulcus angle, congruence angle, lateral patellofemoral angle (LPA), lateral patellar displacement (LPD), lateral displacement measurement (LDM), boss height, and patellar height ratios (Caton-Deschamps, Blackburne-Peel, Insall-Salvati). All assessors were provided with a summary document outlining the method of assessing each measurement. Bland-Altman analyses were adopted to assess intra- and inter-rater reliability. Results. The results indicated generally low measurement error on intra-rater reliability assessment, particularly for LPD (within-subject variance 0.7mm to 3.7mm), LDM (0.7mm to 3.5mm) and boss height (0.4mm to 1.6mm) for all assessors. There was greater measurement error for the calculation of sulcus angle (0.7° to 10.6°), congruence angle (0.8° to 18.4°) and LPA (0.8° to 16.5°). Whilst the inter-rater reliability between assessors indicated a low mean difference for assessments of patellar height measurements (0.0° to 0.6°), there was greater variability for LPA (0.1° to 3.6°), LPD (0.2mm to 4.6mm) and LDM (0.1mm to 4.0mm), with wide 95% limits of agreement for all measurements indicated poor precision. Conclusions. Many of the standard measurements used to assess the patellofemoral joint on plain radiographs have poor precision. Intra-rater reliability may be related to experience but it seems likely that to achieve good inter-rater reliability, specific training may be required to calibrate observers. More formal training in the technique of radiological measurement for those who were inexperienced might have improved the inter-rater reliability

A number of measurements of patella height exist all of which use a position on the tibia as a reference. The Patellotrochlear Index has recently been proposed as a more accurate reflection of the functional height of the patella and described in normal knees. We aimed to compare patellar height measurements in patients with patellofemoral dysplasia. In a retrospective analysis of the MRI scans of 33 knees in 29 patients with patellofemoral dysplasia we assessed the inter- and intraobserver reliability of four patellar height measurements: the Patellotrochlear Index (PTI), Insall-Salvati (IS), Blackburne-Peel (BP) and Caton-Deschamps (CD) ratios. We also assessed the correlation between the different measurements in predicting patella alta. Three blinded observers on two separate occasions performed the measurements. There were 21 females and 8 males with a mean age of 21.4 years (13–33). Statistical analysis revealed good inter-observer reliability for all measurements (0.78 for PTI, 0.78 for IS, 0.73 for BP and 0.77 for CD). Intra-observer reliability was also good (0.80, 0.83, 0.75, 0.78 respectively). When comparing the different measurements for patella alta there was a weak correlation between the PTI and the others. There was a strong correlation between the CD and BP ratios (0.96) and a moderate correlation between IS and CD and IS and BP ratios (0.594 and 0.539 respectively). All measurements are reproducible. The PTI however suggests patella alta exists in different patients to that suggested by the established measures. We propose the PTI as a more clinically relevant measure

A number of measurements of patellar height are in clinical use all of which reference from the tibia. The patellotrochlear index has been proposed recently as a more accurate reflection of the functional height of the patella and described in normal knees. We compared patellar height measurements in patients with patellofemoral dysplasia. In a retrospective analysis of the MRI scans of 33 knees in 29 patients with patellofemoral dysplasia we assessed the inter- and intraobserver reliability of four patellar height measurements: the recently described Patellotrochlear Index (PTI), Insall-Salvati (IS), Blackburne-Peel (BP) and Caton-Deschamps (CD) ratios. We also assessed the correlation between the different measurements in predicting patella alta. Three blinded observers on two separate occasions performed the measurements. There were 21 females and 8 males with a mean age of 21.4 years (13–33). Statistical analysis revealed good inter-observer reliability for all measurements (0.78 for PTI, 0.78 for IS, 0.73 for BP and 0.77 for CD). Intra-observer reliability was also good (0.80, 0.83, 0.75, 0.78 respectively). There was weak correlation between the PTI and the other ratios for patella alta. There was a strong correlation between the CD and BP ratios (0.96) and a moderate correlation between IS and CD and IS and BP ratios (0.594 and 0.539 respectively). We propose the PTI as a more clinically relevant measure

Many radiographic techniques have been described for measuring patellar height. They can be divided into two groups: those that relate the position of the patella to the femur (direct) and those that relate it to the tibia (indirect). This article looks at the methods that have been described, the logic behind their conception and the critical analyses that have been performed to test them.

A number of measurements of patella height exist all of which use a position on the tibia as a reference. The Patellotrochlear Index has recently been proposed as a more accurate reflection of the functional height of the patella and described in normal knees. Aim: A comparison of patellar height measurements in patients with patellofemoral dysplasia. Method: A retrospective analysis of the MRI scans of 33 knees in 29 patients with patellofemoral dysplasia to assess the inter- and intraobserver reliability of four patellar height measurements: the Patellotrochlear Index (PTI), Insall-Salvati (IS), Blackburne-Peel (BP) and Caton-Deschamps (CD) ratios. We also assessed the correlation between the different measurements in predicting patella alta. Three blinded observers on two separate occasions performed the measurements. Results: There were 21 females and 8 males with a mean age of 21.4 years (13–33). Statistical analysis revealed good inter-observer reliability for all measurements (0.78 for PTI, 0.78 for IS, 0.73 for BP and 0.77 for CD). Intra-observer reliability was also good (0.80, 0.83, 0.75, 0.78 respectively). When comparing the different measurements for patella alta there was a weak correlation between the PTI and the others. There was a strong correlation between the CD and BP ratios (0.96) and a moderate correlation between IS and CD and IS and BP ratios (0.594 and 0.539 respectively). Conclusion: All measurements are reproducible. The PTI however suggests patella alta exists in different patients to that suggested by the established measures. We propose the PTI as a more clinically relevant measure

The effect of weight-bearing on the height of the patellar using four radiological indices was studied in 25 healthy men using lateral radiographs of the knee in 30° of flexion non-weight-bearing and weight-bearing. The position of the patella was quantified using the Insall-Salvati, the modified Insall-Salvati, the Blackburne-Peel and the Caton indices. The contraction of the quadriceps on weight-bearing resulted in statistically significant proximal displacement of the patella with all four indices studied. The mean Insall-Salvati index was 0.919 (sd 0.063) before and 1.109 (sd 0.042) after weight-bearing (p = 0.001), while the mean modified Insall-Salvati index was 0.734 (sd 0.039) before and 0.896 (sd 0.029) after weight-bearing (p = 0.041). Similarly, the Blackburne-Peel index was 0.691 (sd 0.09) before and 0.807 (sd 0.137) after weight-bearing (p = 0.012). The mean Caton index was 0.861 (sd 0.09) before and 0.976 (sd 0.144) after weight-bearing (p = 0.023).

The effect of quadriceps contraction should be considered in clinical studies where the patellar position indices are reported.

A radiographic review of sixty-nine lateral closing wedge high tibial osteotomies and forty-two medial opening wedge osteotomies was conducted. Patellar height and tibial slope were measured. The Blackburne-Peel (BP) and Insall-Salvati (IS) ratios were used to measure patellar height. Our results show that 18.8% and 47.6% of the lateral closing wedge group had patella-infera (PI) according the the BP and IS ratios respectively. No opening-wedge cases demonstrated patella infera with either ratio. Tibial slope was found to be significantly more neutral in the closing wedge group versus the opening wedge (−2.2° vs. −7.28° respectively). Patients undergoing HTO typically require a definitive arthroplasty procedure at an average of six years post-HTO. Total knee arthroplasty can be complicated by the presence of PI which compromises exposure and increases the risk of patellar tendon avulsion leading to suboptimal results. The current study compares the incidence of PI between lateral closing and medial opening wedge HTO’s. Conversion of opening wedge HTO to TKA should have less technical challenge and improved outcomes as compared to lateral closing wedge HTO conversions. This is attributable to the absence of PI. Of the closing wedge group, 18.8% and 47.6% were found to have PI according to the BP and IS ratios respectively. Of the opening wedge group there were no cases of PI found with either ratio. Tibial slope was found to be significantly more neutral in the closing wedge compared to the opening wedge group (−2.2° vs. −7.28° respectively). The study cohorts were abstracted from surgeon records. Sixty-nine closing wedge and forty-two opening wedge HTO’s were identified. Lateral 30° flexion x-rays were measured for patellar height and tibial slope. Both the Blackburne-Peel (BP) and Insall-Salvati (IS) patellar height measurements were used. From this data we can conclude that in our study population the incidence of PI in the opening wedge group was 0% thus negating the potential deleterious effects of PI at the time of knee arthroplasty

Postoperative anterior knee pain was evaluated in a consecutive series of 138 knees in 108 patients with rheumatoid arthritis treated by total knee replacement with Mark I Insall-Burstein prostheses. No knee had primary patellar resurfacing, and in the 119 knees followed up for a mean of 63.9 months, none had secondary resurfacing. Anterior knee pain was absent in 87 knees (73%), mild in 16 (13.5%) and moderate or severe in 16 (13.5%). The height of the patella above the prosthetic joint line was the only variable which was directly related to the incidence of anterior knee pain. The sensitivity and specificity of patellar height measurements for identifying patients with or without pain were derived. From these data, a selective policy of resurfacing the patella in those at risk was adopted. Choosing a patellar height of 15 mm or less, patellar resurfacing could be avoided in 80% of patients likely to have no pain, and the patella could be resurfaced in 65% of those likely to have anterior knee pain