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Introduction: Anterior cruciate ligament (ACL) rupture is a debilitating condition and often requires surgery to restore joint stability. Common autografts used for reconstruction include patella tendon and hamstring tendons. The primary aim of this study was to evaluate the early to mid-term clinical outcomes of ACL reconstruction using validated patient-reported outcome measures (PROMs). The secondary aim was to compare clinical outcomes between patella tendon and hamstring tendon autografts. The tertiary aim was to compare clinical outcomes between males and females., Methods: Patients with an ACL rupture were evaluated before and after surgery using PROM scores which included Lysholm, Tegner, International Knee Documentation Committee (IKDC), Knee Injury and Osteoarthritis Outcome Score (KOOS), Short Form-12 Item (SF-12) and EQ-5D-5L., Results: A total of 87 patients were included in the study. All PROM scores significantly improved following surgery (p < 0.001) at a mean follow-up time of 28 months (range 12 to 88 months). The patella tendon subgroup (n = 27) had superior post-operative results as compared to the hamstring tendon subgroup (n = 60) for KOOS sport and recreation (p = 0.005), KOOS quality of life (p = 0.025), KOOS overall (p = 0.026), Tegner (p = 0.046) and IKDC (p = 0.021) scores. There was no significant difference of PROM scores between males (n = 60) and females (n = 27) (p > 0.05)., Conclusions: ACL reconstruction significantly improves clinical outcomes for patients with symptomatic instability consequent to ACL rupture. Overall, patella tendon autograft resulted in better clinical outcomes as compared to hamstring tendon autograft following surgery. Gender did not influence clinical outcome following ACL reconstruction., (© 2024. The Author(s).)

A stress-avulsion fracture of the inferior pole of the patella is rare. We report a case of a 65-year-old woman who underwent open reduction and internal fixation for a transverse fracture of the patella using cannulated screws inserted from the inferior pole of the patella. Subsequently, the patient developed an avulsion fracture of the inferior pole of the patella due to a stress riser from the prominent screw head. The avulsion fracture was treated with open repair and augmentation using a cerclage wire, and the stress riser was eliminated by burying the screw head into the bone. The outcomes were satisfactory. Preventing implant-related stress risers during internal fixation of fractures requires diligent surgical techniques., Competing Interests: Human subjects: Consent was obtained or waived by all participants in this study. Conflicts of interest: In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work., (Copyright © 2024, Rajeev et al.)

Objectives: Autologous tenocyte implantation (OrthoATI™) therapy has demonstrated efficacy in treating patients with tendinopathy at various anatomical sites. This study evaluates the effect of patient age, gender, and tendon biopsy site on morphology, growth, and gene expression of autologous tendon cells used to treat chronic tendinopathy., Methods: Patients undergoing OrthoATI™ for tendinopathies between 2020 and 2022 were initially treated by biopsies taken from patella tendon (PT) or palmaris longus tendon (PL). The biopsies were sent to a Good Manufacturing Practice (GMP) cell laboratory where tendon cells were isolated, cultured, and expanded for four to six weeks. Cell morphology was assessed using phase contrast microscopy. Droplet digital PCR (ddPCR) was utilized for gene expression analysis. Dichotomous results were compared between groups using x 2 or Fisher's exact tests with no adjustment for multiple comparisons. The nonparametric Mann-Whitney U and Kruskal-Wallis tests were utilized for the sex and age (<35y, 35-44y, 45-54y, >55y) analyses, respectively. All analyses were performed using IBM SPSS v27, and a two-tailed P-value of <0.05 was considered statistically significant., Results: 149 patients were included in the analysis. The PT was biopsied in 63 patients, and PL in 86 patients. There were no observer effects for age and gender between the PT and PL groups. There was no statistical significance between the PT and PL tendons for cell morphology, average cell population doubling time (PDT) (PT 83.9 vs PL 82.7 ​h, p ​= ​0.482), cellular yield (PT 16.2 vs PL 15.2 ​× ​10 6 , p ​= ​0.099), and cell viability (PT 98.7 vs PL 99.0%, p ​= ​0.277). Additionally, ddPCR analyses showed no statistical significance found in tenogenic gene expression, including collagen type I (COL1, p ​= ​0.86), tenomodulin (TNMD, p ​= ​0.837) and scleraxis (SCX, p ​= ​0.331) between PT- and PL-derived tendon cells. An age stratification analysis found no effect on growth and gene expression. COL1 was found to be higher in males when compared to females (P ​< ​0.001), but otherwise no difference was seen in growth and gene expression in the gender analysis. No postbiopsy clinical complications were reported for either group., Conclusion: This study has shown that the growth and bioactivities of tendon cells from tendon biopsies for OrthoATI™ are not affected by tendon donor site and age., Level of Evidence: IV., Competing Interests: Declaration of competing interest MHZ & AW hold shares of Orthocell Ltd., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Patellar tendinopathy (PT) is a chronic, degenerative form of tendinitis commonly affecting young, active individuals. Numerous nonsurgical treatments exist, of which platelet-rich plasma (PRP) is a frontrunner. However, heterogeneity among various PRP preparation techniques results in a large variation in treatment efficacy. This review and meta-analysis aims to investigate the effect of PRP centrifugation factors, specifically centrifuge speed and duration, on functional outcomes in patients with PT. A systematic search of the literature was performed in April 2024 on Medline and Embase. Articles involving the use of PRP in the treatment of PT were included. The risk of bias was assessed using the Risk of Bias 2 (RoB 2; the Cochrane Collaboration, England, UK) and Risk of Bias in Non-randomised Studies of Intervention (ROBINS-I; the Cochrane Collaboration, England, UK) tools. Comparative meta-analysis between the different centrifugation speeds and the different centrifugation durations was performed on articles reporting Victorian Institute of Sports Assessment - Patellar tendon (VISA-P) and visual analogue scale (VAS) scores for PT. Seventeen studies consisting of 360 participants were included in the analysis. The mean follow-up duration was 13.2 months (95% CI: 8.81 to 17.7). The mean VAS reduction was 3.85 (95% CI: -4.63 to -3.08; P < 0.01). VISA-P scores improved by 32.03 (95% CI: 24.29 to 39.78; P < 0.01). There were no significant differences between centrifuge speeds for VAS (P = 0.17) and VISA-P (P = 0.18) and between centrifuge durations for VAS (P = 0.25) and VISA-P (P = 0.27). Centrifuge speed and duration and number of centrifuge cycles did not show any significant differences in patient outcomes. There were no significant differences observed in outcomes for the different preparations of PRP. There remains a need for further high-quality RCTs using standardized PRP preparations with long-term follow-up for the development of a consensus method of PRP preparation for the treatment of PT., Competing Interests: Conflicts of interest: In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work., (Copyright © 2024, Zhang et al.)

Purpose: Ultrasound shear wave elastography (SWE) is a tool that can be utilized to assess biomechanical properties of tendons. Anisotropy, an ultrasound imaging artifact has been commonly cited as a potential source of error in the accuracy and reproducibility of SWE. The aim of the study was to assess reproducibility in performing SWE of patella tendons and differences in SWE and anisotropy between normal patella tendons and patellar tendinopathy., Methods: After obtaining the Institutional Review Board approval and written informed consent, we prospectively measured the shear wave velocity (SWV) of patella tendons with and without tendinopathy in 25 volunteers. SWVs were measured in three anatomic planes: longitudinal, perpendicular transverse, and tilted transverse with the probe tilted 15-30° from the perpendicular transverse plane by three operators with varied levels of experience. Anisotropy coefficient (A) was calculated by formula of A = (SWV Longitudinal  - SWV Transverse ) / SWV Transverse ., Results: Differences in SWV and anisotropy coefficient between normal tendons and tendons with tendinopathy were significant (p < 0.05). The intra- and inter-observer reproducibility in performing SWE were moderate to good (intraclass correlation coefficient: 0.81-0.95). The mean difference of 95% Bland-Altman limits of agreement for measuring tendon SWV ranged -0.08 to 0.41 (upper 0.08 to 1.14, lower -1.22 to -0.22) between senior and junior operators., Conclusion: The results of this study suggest that SWE and anisotropy coefficient are feasible tools to differentiate patellar tendinopathy from normal patella tendons. The reproducibility of performing SWE of patella tendons is moderate to good., Competing Interests: Declaration of competing interest Jing Gao, MD received cosigned equipment from Siemens Healthineers to support this study., (Copyright © 2023 Elsevier Inc. All rights reserved.)

The extensor mechanism of the knee can be damaged due to various modes of injury, which, in most cases, will require urgent surgical intervention for repair. Single patellar tendon ruptures are uncommon, but simultaneous bilateral events are even rarer and have been scarcely reviewed in English literature. Research in this area is mainly confined to case series, with some literature reviews but no evidence of more substantial analysis. Therefore, this systematic review was done to analyse the existing literature on bilateral simultaneous patellar tendon ruptures and propose a systematic and standardised approach to diagnosing and managing these injuries. A systematic review was conducted using the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. The search terms included 'bilateral patellar tendon rupture', 'bilateral', 'patellar', 'tendon' and 'rupture'. Three independent reviewers conducted searches in PubMed, OvidSP for Medline, Embase and the Cochrane Library using the same search strategy. The eligibility criteria included studies on bilateral concomitant patellar tendon rupture published in English. Bilateral simultaneous patellar tendon ruptures of traumatic and atraumatic origin in human patients were included. The study types comprised case reports and literature reviews. The key limitation of this study was the low number of patients covered by the eligible literature. Patellar tendon ruptures are a rare and scarcely documented injury, and there is a need for studies with a high level of evidence, especially regarding surgical treatment choice and methods, as well as post-operative management, which could potentially lead to improved outcomes in the management of this injury., Competing Interests: The authors have declared that no competing interests exist., (Copyright © 2023, Fernandes et al.)

Introduction: Lesions of the quadriceps or patellar tendon after total knee arthroplasty (TKA) are a rare but serious complication which, if left untreated, can lead to loss of function of the knee joint. While acute and subacute extensor mechanism disruptions may have several causes, chronic deficiencies are often related to multiple prior revision surgeries for joint infection or aseptic TKA failure. Up to date, biological allograft reconstruction showed unsatisfying results. The use of a monofilament polypropylene mesh is a promising approach for this pathological condition. The aim of the present study was to evaluate clinical, functional and patient reported outcomes of this procedure in patients with chronic extensor mechanism deficiency., Materials and Methods: Twenty-eight patients with chronic extensor mechanism deficiency (quadriceps tendon rupture n  = 9, patellar tendon rupture n  = 19) after TKA were included in this retrospective study. None of the patients were lost to follow-up. Surgical reconstruction was performed at one institution between 2014 and 2020 with a monofilament polypropylene mesh (Marlex Mesh, Bard, Murray Hill, USA). The mean age at the time of surgery was 69 years. Patients presented with a mean BMI of 33 kg/m 2 . The mean follow-up period was 23 months., Results: The 2-year survivorship free of mesh revision was 89% [95% confidence interval (CI): 75% to 100%]. Three patients (11%) had to undergo revision because of mechanical mesh failure and received another polypropylene mesh. No further revisions were performed thereafter. Flexion was 87° (range, 30-120°) on average. The majority of patients (75%, 21/28) had a full active extension. The mean active extension lag after surgery was 4 degrees (range, 0-30°)., Discussion: We observed a substantial improvement of extensor mechanism function. The majority of patients had full extension and showed good clinical results. A failure rate of over 50% has been published for alternative procedures. Thus, the use of the described augmentation technique represents a reasonable treatment option for chronic extensor mechanism disruptions of the patellar tendon as well as the quadriceps tendon after total knee arthroplasty. However, there might be a potentially higher risk for infection persistence in periprosthetic joint infection cases due to the presence of a foreign material., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (© 2022 Fuchs, Gwinner, Meißner, Pfitzner, Perka and von Roth.)

Background: Patella tendinopathy is a common condition in a number of sports and can have a significant impact on activity and quality of life. Whilst rehabilitation leads to improvements in the majority of cases, a number of adjunct treatments are available for resistant cases with limited evidence supporting the use of one treatment over another.Hypothesis/Purpose: To compare the outcomes for patients with chronic patella tendinopathy, following either radial-Extra-Corporeal Shockwave Therapy (r-ESWT) or Autologous Blood Injection (ABI)., Study Design: Prospective cohort study., Methods: 28 consecutive patients with chronic patella tendinopathy, treated with radial-ESWT (n = 14) or ABI (n = 14), with minimum three-month follow-up. Mean age 34.2 years, mean duration of symptoms 32.8 months. Patients received either three sessions of radial-Extra-Corporeal Shockwave Therapy (one session per week for 3 weeks), or two ultrasound-guided Autologous Blood Injections (2 injections performed 2-4 weeks apart). All patients received standardised after-care, including continuation of a structured home exercise programme of flexibility and progressive loading (eccentric strengthening) exercises. Main Outcome Measures were Victoria Institute of Sport - Patella questionnaire (VISA-P), plus 0-10 Visual Analogue Scale (VAS) for self-reported levels of "average pain"., Results: There were statistically significant improvements in self-reported measures of pain/function following either procedure at 6-weeks and at 3-months, but only in the ABI group at 6-months. At 3-months: VISA-P was improved from 32.4% ± 11.7%-60.0% ± 20.7% (p < 0.01) in the ABI group, and from 34.2% ± 14.6%-48.9% ± 17.8% (p < 0.001) in the r-ESWT group., Conclusions: Patients with chronic patella tendinopathy improved at 6-weeks and 3-months following either ABI or r-ESWT procedure, with significant benefits at 6-months seen only in the ABI group. Between group analysis demonstrated improved outcomes favouring ABI compared to r-ESWT at the 6-month timepoint using VISA-P, and at all time-points using VAS as outcome measures. The small sample sizes in this pragmatic study are however noted, which limit interpretation, and larger more robust studies are required to investigate this further. This pragmatic prospective cohort study demonstrates improvements following either r-ESWT or ABI procedures, with results potentially favouring ABI over r-ESWT for patients with chronic patella tendinopathy., Competing Interests: The author states that there is “No other relationships/conditions/circumstances that present potential conflict of interest”, (© 2022.)

Ruptures of the extensor mechanism of the knee are serious injuries requiring prompt diagnosis and treatment. They can be divided into soft tissue and bony causes. Soft tissue tendon injuries can be either partial or complete. Rupture of the quadriceps tendon is an uncommon injury and is more frequent in patients over the age of 40 years. Patella tendon ruptures are even rarer and are more frequent in patients under the age of 40 years. Causes can be direct or indirect. Complete ruptures of the quadriceps tendon or patella tendon benefit from early surgical management, while partial ruptures may be managed non-operatively. This article gives an overview of the presentation, assessment and management of soft tissue extensor mechanism tendon ruptures for core surgical, acute care common stem and emergency medicine trainees.

Purpose: To evaluate if High-volume Image-guided Injection (HVIGI)-treatment for chronic Patellar tendinopathy (PT) improve function and reduce pain at 16-months follow-up., Methods: Patients with resistant PT who failed to improve after a three-month eccentric loading program were included in the study. Maximal tendon thickness was assessed with ultrasound. All patients were injected with 10 mL of 0.5% Marcaine, 0.5 mL Triamcinolonacetonid (40 mg/mL) and 40 mL of 0.9% NaCl saline solution under real-time ultrasound-guidance and high pressure. All outcome measures were recorded at baseline and at 16 months. A standardised Heavy Slow Resistance rehabilitation protocol was prescribed after HVIGI-treatment. Clinical outcome was assessed with the Victorian Institute of Sports Assessment-Patella tendon questionnaire (VISA-P) and statistically analyses were performed., Results: The study included 28 single treatment HVIGI procedures in PT in 23 patients (19 men, 4 women) with a mean age of 30.3 (range 19-52). The mean duration of symptoms before HVIGI was 33 months. The baseline VISA-P score of 43 ± 17 (range 15-76) improved to 76 ± 16 (range 42-95) after 16 months (p < 0.01). Of the 28 HVIGI procedures 12 patients (15 PT) were not satisfied after the initial HVIGI procedure. Of these, 5 patients (5 PT) had additional HVIGI, 2 patients (2 PT) had corticoid injection and 6 patients (8 PT) needed surgery. Of the remaining 11 patients (13 PT), 9 patients had more than a 13-point improvement in the VISA-P score after 16 months., Conclusions: In this retrospective case-study, only 9 patients (32%) did benefit of a single HVIGI treatment at 16-months and a 33-point significant improvement was seen on the VISA-P score.

Objectives: To determine normal temperatures over the Patella tendon over eleven weeks., Design: A prospective cohort study with eleven weeks of observation., Setting: University's Human Biomechanics and Physiology Laboratory., Participants: Male or female collegiate runners running at least 25 miles per week who did not report pain in the region of the Patella tendon over 11 weeks of data collection., Main Outcome Measures: Thermal images taken at the same time and day of the week, were used to measure the temperature of the skin over the Patella tendon., Results: Eighteen athletes were eligible for analysis. The mean temperature of the Patella tendon was 30.13 °C (SD = 1.51 °C). Patella tendon temperature changes over time were insignificant (right p = 0.66, left p = 0.90) with ICC right = 0.92, left = 0.94. Mean temperature difference side to side was 0.14 °C (SD = 0.60 °C). Mixed-model Linear regression for mean temperature differences found the effect of (i) time (t = 0.39, p = 0.70, df = 361) and (ii) side (t = -0.89, p = 0.38, df = 361) to be insignificant., Conclusions: This is the first report of normal thermal profiles of collegiate runners over an extended period. Temperature variation above 1.20 °C may represent an abnormal asymmetry in the running population. Variations in Patella tendon temperatures left to right, and over time were not significant., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Achondroplasia is a genetic mutation of fibroblast growth factor receptor resulting in impaired growth plate development in long bones due to lower collagen turnover. Despite the characteristic shorter stature and lower strength in Achondroplasic groups, little is known of the tendon mechanical properties under loading. The aim of this study was therefore to conduct a between measure design of patella tendon (PT) mechanical properties (stress, strain, stiffness and Young's Modulus) in 10 men with Achondroplasia (22 ± 3 years) and 17 male controls (22 ± 2 years). PT mechanical properties were measured during isometric maximal voluntary contraction (iMVC) of the knee extensors using ultrasonography. The Achondroplasic group produced 54% less stress at iMVC than controls (29.4 ± 8.0 v 64.5 ± 14.0 MPa, P < 0.001, d = 3.12). Maximal excursion of the Achondroplasic PT was 22% less than controls at iMVC (7.4 ± 2.1 v 5.5 ± 1.7 mm, P < 0.001, d = 0.99), but there was no difference in strain between groups (13 ± 4 v 13 ± 3%, P > 0.05). Achondroplasic PT were 47% less stiff (748 ± 93 v 1418 ± 101 N·mm -1 , P < 0.001, d = 6.89) and had a 51% lower Young's modulus (0.39 ± 0.09 v 0.77 ± 0.14 GPa, P < 0.001, d = 3.46) than controls at iMVC. Achondroplasic PT are indeed more compliant than controls which may contribute to lower relative force production. The causes of higher Achondroplasic PT compliance are unclear but are likely due to the collagen related genetic mutation which causes Achondroplasia.

To examine and compare the loading related changes in micro-morphology of the patellar tendon. Fifteen healthy young males (age 19±3yrs, body mass 83±5kg) were utilised in a within subjects matched pairs design. B mode ultrasound images were taken in the sagittal plane of the patellar tendon at rest with the knee at 90° flexion. Repeat images were taken whilst the subjects were carrying out maximal voluntary isometric contractions. Spatial frequency parameters related to the tendon morphology were determined within regions of interest (ROI) from the B mode images at rest and during isometric contractions. A number of spatial parameters were observed to be significantly different between resting and contracted images (Peak spatial frequency radius (PSFR), axis ratio, spatial Q-factor, PSFR amplitude ratio, and the sum). These spatial frequency parameters were indicative of acute alterations in the tendon micro-morphology with loading. Acute loading modifies the micro-morphology of the tendon, as observed via spatial frequency analysis. Further research is warranted to explore its utility with regard to different loading induced micro-morphological alterations, as these could give valuable insight not only to aid strengthening of this tissue but also optimization of recovery from injury and treatment of conditions such as tendinopathies., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Background and Aims: Quadriceps and patella tendon ruptures are uncommon injuries often resulting from minor trauma typically consisting of an eccentric contraction of the quadriceps muscle. Since rupture of a healthy tendon is very rare, such injuries usually represent the end stage of a long process of chronic tendon degeneration and overuse. This review aims to give an overview of the current understanding of the pathophysiology, diagnostic principles, and recommended treatment protocols as supported by the literature and institutional experience., Material and Methods: A non-systematic review of the current literature on the subject was conducted and reflected against the current practice in our level 1 trauma center., Results and Conclusion: Risk factors for patella and quadriceps tendon rupture include increasing age, repetitive micro-trauma, genetic predisposition, and systemic diseases, as well as certain medications. Diagnosis is based on history and clinical findings, but can be complemented by ultrasound or magnetic resonance imaging. Accurate diagnosis at an early stage is of utmost importance since delay in surgical repair of over 3 weeks results in significantly poorer outcomes. Operative treatment of acute ruptures yields good clinical results with low complication rates. Use of longitudinal transpatellar drill holes is the operative method of choice in the majority of acute cases. In chronic ruptures, tendon augmentation with auto- or allograft should be considered. Postoperative treatment protocols in the literature range from early mobilization with full weight bearing to cast immobilization for up to 12 weeks. Respecting the biology of tendon healing, we advocate the use of a removable knee splint or orthotic with protected full weight bearing and limited passive mobilization for 6 weeks., (© The Finnish Surgical Society 2015.)

This case report describes a 12-year-old boy, who suffered an injury to the right knee in a skateboard accident. Radiographs and surgery confirmed the extremely rare bifocal avulsion fracture including the distal patellar pole and tibial tuberosity. Open reduction and internal fixation was accomplished, and 4-month follow-up demonstrated a good outcome.

We describe a series of three patients who sustained patella tendon injuries in infected total knee arthroplasties following the use of a static cement spacer at first-stage knee revision. The patella tendon injuries resulted in significant compromise to wound healing and knee stability requiring multiple surgeries. The mid-term function was poor with an Oxford score at 24 months ranging from 12 to 20. Based on our experience, we advise caution in the use of static cement spacer blocks. If they are to be used, we recommend that they should be keyed in the bone to prevent patella tendon injuries.

Patellar tendon avulsion is a risk with difficult exposure in a stiff knee, patella baja or previous tibial osteotomy. We sought to define a safe amount of release of the patellar tendon insertion for such cases. Eleven pairs of fresh frozen cadaveric lower limb specimens were acquired and randomized to either intact or partial release of the distal tibial insertion. Partial release of the tibial insertion of the tendon increased lateral exposure a mean 29% ± 15% (P = 0.002) while reducing ultimate strength to a mean of 80% that of the intact contralateral tendon. Measured patella release increased lateral patella translation and can be performed without risk of catastrophic rupture with basic activities of daily living following TKA., (© 2014.)

There is much literature about differing grafts used in anterior cruciate ligament (ACL) reconstruction. Much of this is of poor quality and of a low evidence base. We review and summarise the literature looking at the four main classes of grafts used in ACL reconstruction; bone-patella tendon-bone, hamstrings, allograft and synthetic grafts. Each graft has the evidence for its use reviewed and then compared, where possible, to the others. We conclude that although there is no clear "best" graft, there are clear differences between the differing graft choices. Surgeon's need to be aware of the evidence behind these differences, in order to have appropriate discussions with their patients, so as to come to an informed choice of graft type to best suit each individual patient and their requirements.

Post-operative patellar tendon shortening induces a distal positioning of the patella in the femoral trochlear groove, which has been associated with pain and impeded mobility. An idealized in vitro model was used to examine the effects of shortening on patellar kinematics. The PT length was progressively reduced by up to 5 mm (1-mm instalments) using a device secured onto the tendon in n = 9 ovine stifles. In vitro 6 degrees-of-freedom motion data for the patellofemoral and tibiofemoral joints under conditions of passively induced flexion-extension was acquired electromagnetically. Patellar motion was analysed as a function of both tibial and patellar flexion angles relative to the femoral co-ordinate frame. Linear regression with contrasts was used to compare kinematic changes for each shortening level, with significance set at P<0.01. A mean maximum percentage length reduction of 8.2% was achieved. Patellar flexion was linearly correlated with tibial flexion angle in the intact joint, and this correlation persisted after tendon shortening (R = 0.977, P < 0.01). Patellar kinematics expressed as a function of tibial flexion angle were significantly altered by a mean length decrease of 8.2%, while flexion and proximo-distal shift patterns were significantly affected at lesser shortening levels of 3.1% and 4.7%, respectively. Patellar kinematics expressed as a function of patellar flexion angle remained unchanged. These results suggest that patellar motion within the trochlear groove in the ovine stifle joint follows a repeatable three-dimensional path and that patellar tendon shortening advances the position of the patella along this path, without significantly altering it.