Patellar Stress Research Articles

No reduced patellar loading with latest-generation cruciate-retaining total knee arthroplasty\u2014a comparative study of Attune and Press-Fit Condylar\xae

PurposeTo investigate if the latest-generation cruciate-retaining total knee arthroplasty (CR-TKA) systems through more patella-friendly femoral trochlea reduce the patellar bone loading.MethodsTwenty patients who underwent Attune CR-TKA were matched with twenty-one patients who underwent Press-Fit Condylar® (PFC) CR-TKA. The patella was always preserved. The in vivo patellar loading was measured twice by two blinded observers and localised on an 8-quadrant grid on 1-year post-operatively SPECT/CT images. The position of the TKA components, patella height, thickness, tilt, and tibial tuberosity-trochlear groove index were measured in 3D CT. Knee function was assessed pre-operatively, at 12 and 24 months post-operatively with the knee society score (KSS). All data were compared between groups with the Mann-Whitney U test and within groups with Spearman’s correlation.ResultsA significantly higher bone tracer uptake (BTU) was seen in the Attune group in the lateral non-articular patellar quadrants. No other significant differences of the BTU were seen. The post-operative KSS did not differ significantly. Spearman’s correlation showed no correlations between the significantly higher BTU of the lateral non-articular patellar quadrants and the position of the TKA and patellar measurements. All patellar measurements did not correlate with bone stress in SPECT/CT.ConclusionNo significant improvement in terms of in vivo patellar bone stress was seen with the latest-generation CR-TKA system. The increased bone stress at the non-articular lateral patellar quadrants of the Attune could be due to higher stabilising quadriceps forces.

The dysplastic trochlear sulcus due to the insufficient patellar stress in growing rats

BackgroundDevelopmental factors were assumed to be the key factors that influenced the morphology of femoral trochlea. This study investigated the effects of insufficient patellar stress after birth on the morphological development of the femoral trochlea. Effects of insufficient patellar stress on femoral trochlea were investigated using surgical induced patellectomy and patellar dislocation in growing rat model.MethodsIn this study, two experimental groups and one sham group (SG) were established. Thirty-six Wistar rats (female, 28 days of age) were randomly assigned to three groups. The patellectomy group (PG), rats underwent the patellectomy in this group. The dislocation group (DG), rats underwent the surgery-induced patellar dislocation. Histological staining (Safranin-O and fast green), Micro-computed tomographic (Micro-CT) analysis in two experimental endpoints (3, 12 weeks postoperatively) were selected to evaluate morphological changes of the femoral trochlea.ResultsArticular cartilage on the trochlear sulcus was remodeled at 3 weeks after the surgery, and degenerated at 12 weeks through the histological staining. The femoral trochlear angle (FTA) did not show a significant difference at 3 week between the experimental groups and the sham group (PG vs SG P = 0.38, DG vs SG p = 0.05), but the FTA was significantly increased in experimental groups at 12 weeks(PG vs SG P = 0.001, DG vs SG p = 0.005). The Bone volume density (BV/TV), Trabecular number (Tb.N) under the trochlea groove were significantly reduced at 3 weeks postoperatively in the experimental groups (PG vs SG p = 0.001, DG vs SG p = 0.002). No significant difference was found in BV/TV and Tb. N among the three groups at 12 weeks postoperatively.ConclusionSurgical induced patellectomy and patellar dislocation leads to the dysplastic trochlear sulcus in growing rats. Besides the bone morphology of trochlear sulcus, the articular cartilage and subchondral trabecula under the trochlear sulcus were remodeled early stage after the surgery.

Computer Assisted Mechanical Axis and Kinematic TKA

Introduction:Total knee arthroplasty (TKA) has traditionally been and largely continues to be aligned mechanically, that being with a neutral coronal plane mechanical tibiofemoral axis and a joint line orientated at 900 to this axis. Femoral component rotation is set by gap balancing or by externally rotating 30 from any of a number femoral reference lines. This produces a rectangular flexion gap and relaxes patellar tracking. Kinematic alignment (KA) is an alternative technique that aims to restore premorbid alignment, joint orientation and ligament tension. The basic premise for this technique is based on evidence that the medial and lateral femoral condyles consistently equate to cylinders of equal or near equal size and that therefore with a fixed radius, cruciate retaining implant, matched distal femoral, posterior femoral and proximal tibial resections, accounting for bone and cartilage already lost will reproduce the premorbid joint line and restore native premorbid kinematics. Femoral rotation is therefore referenced off the prearthritic posterior condylar axis (PCA) that is on average internally rotated to the AP axis. Kinematic alignment therefore has the potential to challenge patellar tracking, increase patellar load and potentially increase patellar complications.Method:Case control study – level of evidence III-2. Between November 2012 and June 2013 the senior author completed 104 consecutive computer assisted (CAS) kinematically aligned total knee arthroplasties (TKA) with a cruciate retaining, fixed bearing, single radius implant. The results of these surgeries were compared with the results of 91 consecutive CAS mechanically aligned TKA done between November 2011 and October 2012 using the same navigation system and implant Implant sizing and positioning as well as gap measurement and ligament balance was done with computer assistance in all cases. Data was collected prospectively and analysed retrospectively.Results:The Oxford Knee Score improved from 22.2 preoperatively to 42.3 postoperatively in the kinematic group and from 21.5 to 42.1 in the mechanical group. Improvements in the KOOS domains were similar between the two groups except for the sport domain that favoured the kinematic group. Postoperative range of motion was slightly better in the mechanical group. On average there was a difference of femoral component rotation of 4.50. In the kinematic group femoral position was 2.30 internally rotated relative to the AP axis (range 10.50 IR to-6.50 ER) whereas in the mechanical group femoral position was 1.20 externally rotated (range 50 IR to 90 ER. There were six patellofemoral complications in five patients in the kinematic group requiring four additional procedures. Complications included patellar stress fracture / AVN in four patients, one acute patellofemoral dislocation and one chronic patellofemoral dislocation. There were no patellofemoral complications in the mechanical group.Conclusions:The overall results of CAS kinematic and mechanical TKA are similarly excellent but patellofemoral complications occur at an unacceptably high rate with strict kinematic positioning. Patellar resurfacing should be undertaken with caution in kinematic TKA. In the context of kinematic conflict between the posterior condylar and AP axes the femoral component should be externally rotated to reduce load on the patella.

Quantitative stress radiography of the patella and evaluation of patellar laxity before and after lateral release for recurrent dislocation patella

The aims of the present controlled clinical study were to (1) compare patella laxity determined in the outpatient clinic with that in anaesthetized patients and (2) evaluate patella laxity before and after lateral release. The study evaluated data on 33 knees from 33 patients (average age 19.7years) between 2007 and 2011. All patients were diagnosed with recurrent dislocation of the patella. Patellar stability was evaluated in each patient thrice: patellas were first imaged in the outpatient clinic prior to surgery at 45° knee flexion with 20N stress from the medial to lateral side and from the lateral to medial side; then, at the time of surgery, patella stress images were obtained in the same manner before and after the lateral release procedure. Radiological assessments were performed using the medial stress shift ratio (MSSR) and lateral stress shift ratio (LSSR). There were no significant differences in the LSSR and MSSR before surgery (outpatient data) and in anaesthetized patients before the lateral release procedure. Furthermore, there was no significant difference in MSSR at the time of surgery before and after the lateral release procedure. However, LSSR increased significantly after the lateral release procedure. The results of the present study suggest that quantitative patella stress radiography in the outpatient clinic is useful when it comes to investigating laxity of the patella, and that lateral release significantly increases lateral, but not medial, laxity in patients with recurrent patellar dislocation. IV.

Suture Anchor Repair of Quadriceps Tendon Rupture

Quadriceps tendon rupture is an uncommon knee injury that warrants surgical treatment. Traditional surgery has been relatively successful, using trans-patellar-drilled suture repair for tendo-osseous avulsions and simple suture repair for intra-tendinous midsubstance tears. However, recent evidence has identified a unique complication previously unreported in the literature in these patients (displaced longitudinal patellar stress fracture). Suture anchor-based repair has biomechanical strength equivalent to trans-patellar-drilled repair and avoids the potential complication of postoperative fracture. A “pulley technique” with complete footprint coverage has been used for other tendinous avulsions (eg. proximal hamstring) and is described in this article. [ Orthopedics. 2014; 37(3):183–186.]

Patellar calcar: MRI appearance of a previously undescribed anatomical entity

The femoral calcar is a constant anatomical structure within the proximal femur representing a condensation of bone trabeculae. It is our impression that a similar structure is present within the patella. The purpose of this retrospective study was to define the prevalence, appearance, location, and configuration of the patellar calcar on MRI examinations. One hundred consecutive unenhanced knee MRIs were retrospectively reviewed by two readers who were blinded to the clinical indication. The patellar calcar was defined as a dark signaling, linear or curvilinear structure subjacent to the patellar articular surface. If present, the patellar calcar was assigned to a "well seen," "moderately well seen," or "faintly seen" category. Location of the calcar within the patella, orientation, configuration, and thickness were recorded. Confounding variables, such as marrow edema, patellar chondromalacia, bipartite patella, or postoperative changes were also recorded. The patellar calcar was visualized in 81 out of 100 (81%) MRIs. When detected, the calcar was well seen in 20 out of 81 (25%), moderately well seen in 35 out of 81 (43%), and faintly seen in 26 out of 81 (32%). The anteroposterior width of the calcar measured at its thickest segment was: < 1mm in 43 out of 81 (53%), 1mm in 28 out of 81 (35%), and >1mm in 10 out of 81 (12%). The patellar calcar was seen in the majority of knee MRIs and had a consistent imaging appearance. The calcar may be obscured by degenerative arthrosis of the patella and rarely may mimic patellar stress fracture or osteochondritis dissecans. Radiologists and clinicians should be familiar with this normal anatomical structure.

Finite Element Analysis of Resurfacing Depth and Obliquity on Patella Stress and Stability in TKA

Patella resurfacing in total knee arthroplasty (TKA) reduces postoperative complications and revisions; however, the optimal cutting depth and angle that minimize patellar strain and fracture remain unclear. We performed three-dimensional finite element analysis (FEA) of resurfacing cutting depth and obliquity to assess the stresses in each component of the knee joint, and fatigue testing to determine cyclic loading conditions over the expected life span of the implant. Maximum stress on the patella increased as cutting depth increased up to 8mm; peak stresses on the idealized button further increased at 10-mm depth. Medial superior obliquities below 3° showed the lowest stress on the patella and button and the highest fatigue life. An oblique cut of 3° with respect to the inferior end increased patellar stress and reduced fatigue life, making this the least successful approach. Taken together, our FEA supports the use of minimal cutting depths at −3° with respect to the superior end for patellar resurfacing in TKA in order to minimize stresses in the structure and improve TKA durability. Future studies will assess the effect of patella button placement to account for real-world practice variations.

A case of patellar fractures in monozygotic twin gymnasts

We present a case of near identical patellar fractures in adolescent monozygotic twins who are both high-level competitive gymnasts. These patients presented 14 months apart with almost identical history and clinical findings. Both had an intense training regime involving over 30 hours per week of load-bearing exercise. Clinical and radiological examinations suggested avulsion or sleeve fracture of the inferior pole of the patella with minimal displacement. Diagnoses of patellar stress fracture with avulsion of the distal pole and symptomatic bipartite patella could not be reliably excluded. Both fractures were treated conservatively with immobilisation of the knee in extension. An excellent functional result was observed in both patients with return to full activity at 8 weeks.This is the first published case of identical injury to the patella in monozygotic twins. A significant genetic influence on bone mineral density (BMD) has been reported and low BMD is associated with increased susceptibility to fracture. These injuries corroborate a genetic influence on susceptibility to fracture. There is a requirement for further work to investigate genetic factors influencing susceptibility to fracture.

Patellar Stress Fracture After Transosseous Extensor Mechanism Repair

Extensor mechanism injuries are unusual and require early operative intervention to optimally restore extensor mechanism continuity, strength, and function. Quadriceps tendon ruptures typically occur in patients older than 40 years and can be associated with degenerative tendon changes, obesity, systemic illnesses, and trauma. Patellar tendon ruptures are usually seen in younger patients as a result of eccentric trauma. In the acute setting, primary repair is usually undertaken. For tears within the quadriceps or patellar tendon substance, direct repair can be performed, with or without augmentation. When the rupture occurs at the junction of the tendon and the patella, transosseous sutures or suture anchors can be used to reapproximate the tendon-bone interface. Stress fractures of the patella are reportedly rare. They can be seen in association with knee arthroplasty, anterior cruciate ligament (ACL) surgery, and rarely after impact sports. The fracture pattern is usually transverse. Longitudinal stress fractures do not typically disrupt the extensor mechanism and have not, to our knowledge, ever been identified as a postoperative complication of extensor mechanism repair. One instance of a traumatic transverse patellar fracture and extensor mechanism disruption after patellar tendon repair has been described, and a transosseous suture technique was used in that case. No instances of patellar fracture after quadriceps tendon repair have been reported. We report 3 cases of longitudinal patellar stress fractures after extensor mechanism repair using a transosseous suture technique, 2 of which occurred after quadriceps tendon repair and 1 of which occurred after patellar tendon repair.

Does improving vastus medialis function reduce lateral patellar stress in people with PFJ OA?

Introduction: Knee osteoarthritis (OA) affects almost one-third of adults aged over 65 years and is associated with significant personal and economic burden. The patellofemoral joint (PFJ), one of the three knee joint compartments, is the focus of this research since it is frequently affected by symptomatic OA and contributes significantly to the morbidity associated with the disease. The lateral portion of the PFJ experiences greater forces than the medial, and this reflects the disease pattern in this joint, with the lateral compartment being predominantly affected by the OA process.

Mid-Term Survivorship Results for a Rotating-Platform Knee Prosthesis

Interest in mobile-bearing knee prostheses is increasing in the US market. We studied results at 2 to 5 years with a mobile-bearing system that includes a cobalt-chrome tibial tray and femoral component with a polyethylene cruciate-retaining tibial component insert that allows rotation around a central axis and can be used with cruciate-retaining or posterior-stabilized femoral components. The inserts used in this study were cruciate retaining and did not include the posterior-stabilized design. The goal of this study was to demonstrate the function and safety of this prosthesis along with the lack of spinout, which is a major concern in the mobile-bearing knee. Four hundred thirty-five knees constituted the study cohort and underwent survivorship analysis and complication reporting. Routine clinic evaluations included pre- and postoperative radiographs and Knee Society knee and function scores at 6 and 12 weeks and every 2 years. The most recent follow-up data within 2 to 5 years was included for the study along with survey data. Flexion at most recent follow-up averaged 125°. Knee Society score at most recent visit averaged 88 of 100. Knee Society function score averaged 83 of 100. Radiographic results were available for 226 knees, with 97.3% assessed as normal and 6 with these issues: patella stress fracture (3), aseptic tibial loosening (1), patellar osteolysis (1), and patella aseptic loosening (1). In comparison with the fixed-bearing knee equivalent, this mobile-bearing knee demonstrated at least equivalent results in terms of survivorship, function, and patient satisfaction in the short- and mid-term.

Recent advances in understanding the biomechanics of patellofemoral pain

While much of the major clinical and research interest in patellofemoral pain (PFP) has been targeted to the local knee muscles, it has long been recognized that proximal (hip and pelvis) biomechanics may be impaired in people with PFP. This clinical observation has led to recent research in pelvis and hip muscle contributions to PFP. Hip muscles (particularly the abductors and external rotators) are important in maintaining an optimal lower limb alignment during weight bearing activities. A growing body of contemporary evidence indicates that hip muscle function is compromised in PFP. This is highlighted by a recent systematic review, which found strong evidence for deficits in hip muscle strength (abduction, external rotation, extension) in women with PFP compared to uninjured controls. We have observed a delayed onset of gluteus medius electromyographic (EMG) activity in people with PFP compared to healthy controls, confirming earlier findings. Reduced strength or neuromotor control of these hip muscles may be associated with an increase in hip internal rotation and adduction, with deleterious consequences at the knee. Notably, altered proximal biomechanics (hip and pelvis) can influence local patellar alignments and joint stress. Thus, the available evidence supports altered proximal biomechanics as an important feature of PFP, and has provided impetus for contemporary clinical management, favouring hip muscle retraining. However, there is a dearth of clinical trials investigating the clinical efficacy of hip muscle retraining in PFP. Similar to vasti muscle dysfunction, individuals with hip muscle dysfunction form a subgroup of people with PFP. Interventions designed to enhance hip muscle function are likely to benefit patients with PFP. Further scientific evidence is required to confirm the role of hip muscle dysfunction in the development and management of PFP.

Displaced Longitudinal Stress Fracture of the Patella

Stress fractures often occur in physically active individuals as a result of repetitive strenuous muscle and tendon forces acting on bones that have not adapted to such forces1-3. To our knowledge, the literature contains no reports of displaced longitudinal stress fractures of the patella requiring open reduction and internal fixation. The quadriceps muscle and patellar tendon produce extensive forces that can result in a displaced transverse patellar stress fracture2-7. The less strong vertical patellar restraints, including the medial patellofemoral ligament8, produce most of the ligamentous patellar stability in the lateral direction near knee extension8. In cases of acute traumatic patellar dislocation, the osseous structure of the patella is compromised mostly in the lateral direction—e.g., by avulsion of the medial patellofemoral ligament insertion—causing a medial margin fracture of the patella9. We are not aware of any previous reports of repetitive stress causing a displaced longitudinal patellar stress fracture. Here, we describe a case of a displaced longitudinal stress fracture of the patella in a female athlete. The patient was informed that data concerning the case would be submitted for publication, and she consented. An eighteen-year-old female athlete who played floorball (a popular European team sport that is a variant of floor hockey) presented with chronic anterior knee pain that had begun a few months previously without any specific injury event. She was an international-level athlete, and her training had become more difficult because of the persistent anterior knee pain; however, she continued to exercise at a reduced intensity. At the first visit to an orthopaedic surgeon, radiographs were considered to show normal findings and clinical examination revealed no specific findings other than anterior knee pain with some tenderness of the patella. She was diagnosed with nonspecific anterior …

Iliotibial band release as an adjunct to the surgical management of patellar stress fracture in the athlete: a case report and review of the literature

Stress fracture of the patella is rare. In this report, a case of patellar stress fracture occurring in an amateur athlete is presented, and an operative adjunct to the surgical management of this condition is proposed.A review of the English literature identified 21 previous cases of stress fracture of the patella, the majority in young athletes. None of these reports discussed treatment addressing the pathological process contributing to patellar stress fracture.The subject of this case report is a young male netballer who presented with a transverse stress fracture in the inferior third of his patella, on a background of patellofemoral overload. The patient underwent open reduction and internal fixation of his patella, combined with release of the iliotibial band. He returned to training after 6 weeks.The previous literature suggests that operative fixation is indicated for the treatment of displaced patellar stress fractures. Iliotibial band release, as a surgical adjunct to this treatment, may address the pathology of these fractures, and facilitate a return to sport at the highest level.

The Management of Patella Stress Fractures and the Symptomatic Bipartite Patella

Stress fractures of the patella are uncommon. If patellae displace, operative treatment and prolonged rehabilitation will be required and, therefore, a high degree of suspicion is needed. Patients typically report activity-related pain (as a result of an initial stress response) and then subsequently sustain a fatigue stress fracture after minor trauma. In cases where the patella has been previously weakened, such as after patella resurfacing, the term insufficiency stress fracture is used. Diagnosis is made with radiographs and a bone scan and MRI when necessary. Treatment is based on the extent of the fracture, whether complete or incomplete, and the displacement. Incomplete fractures or a stress response may be treated with activity modification alone, complete undisplaced fractures require immobilization until healing occurs, and displaced fractures should have operative reduction and internal fixation to restore natural biomechanics. The symptomatic bipartite patella may also present with anterior knee pain. Those that fail nonoperative measures may require surgical excision. Overall, patients can be expected to make a full recovery and return to preinjury levels of activity.

Overload syndromes of the knee in adolescents: Sonographic findings

Overload syndromes are caused by repetitive microtrauma, and the knee joint is most frequently affected in adolescents. The reason for this is that the knee joint is engaged in almost all sports activities. Pathologies related to the anterior aspect of the knee are: femoropatellar pain, jumper's knee syndromes, Osgood–Schlatter disease, Sinding-Larsen–Johansson syndrome and patellar stress fractures; to the medial aspect: semimembranous tendon enthesopathy and pes anserinus bursitis; to the lateral aspect: iliotibial band syndrome (runner's knee), popliteus and femoral biceps tendon enthesopathy; to the posterior aspect: fabella syndrome and medial gastrocnemius muscle tendon enthesopathy. Sonography plays a central role in the diagnosis and can also evaluate the evolution of diseases. This method is well accepted by the patients and by their parents, it does not involve exposure to X-rays and it is inexpensive. US imaging should, therefore, be considered a first-line imaging diagnostic technique in functional overuse syndromes of the knee.

Isolated Paget Disease of the Patella Complicated with Ipsilateral Femoral Head Bone Marrow Edema: A Case Report and Review of the Literature

The patella is rarely affected by Paget disease of bone. The authors report a 54-year-old lady with ipsilateral knee and hip pain, initially misdiagnosed as patella stress fracture and early degenerative hip arthritis, who was later found to have Paget disease of the patella with ipsilateral femoral head bone marrow edema.

KNEE PAIN-WEIGHT LIFTING

HISTORY A 22 years old female elite olympic weight lifter presented with acute pain and swelling of the right patella after forcefully flexing the knee while lifting. She had a history of several months of knee pain associated to training and competition treated with modification of activity, physical therapy, and antiinflamatory medications. She denied trauma, menstrual irregularities or recent weight loss. PHYSICAL EXAMINATION The exam revealed swelling over the distal anterior portion of the right patella. She exhibited tenderness to palpation bilaterally over the distal patella but more significantly in the right side. She had full knee motion but exhibited pain with resisted knee extension. The exam did not reveal any patellar instability or ligamentous laxity. DIFFERENTIAL DIAGNOSIS Patellar Fracture. Patellar Tendinosis. Patellofemoral Pain. Patellar Subluxation. TEST AND RESULTS X-Rays: Undisplaced right inferior pole patellar fracture -Bone Scan: Stress fracture right patella, stress reaction left patella and right tibial tuberosity -MRI: Transverse horizontal fracture right patella FINAL/WORKING DIAGNOSIS Right Patellar Stress Fracture. Left Patellar Stress Reaction. Patellar Tendinosis. TREATMENT AND OUTCOMES Open reduction internal fixation and bone grafting right patellar fracture. Debridement right patellar tendinosis. Early weight bearing with crutches. Immediate rehabilitation and early motion. Progressive strengthening and functional rehabilitation.

The Insall-Burstein II prosthesis: A 5- to 9-year follow-up study in osteoarthritic knees

We prospectively studied 100 Insall-Burstein Posterior Stabilized II total knee arthroplasties (Zimmer Inc, Warsaw, IN) consecutively implanted in 91 patients with osteoarthritis. We reviewed 92 knees in 85 patients at a mean follow-up of 7.5 years (range, 5.3–9.5 years). According to the Knee Society score, 78 (85%) results were excellent, 11 (12%) were good, and 3 (3%) were fair or poor. The unsatisfactory results were attributed to patellofemoral pain with a low patella (2 knees) and to valgus alignment and medial laxity (1 knee). Flexion at follow-up averaged 116°. Nonprogressive radiolucent lines were present around 30% of the tibial components. There was only 1 small osteolytic lesion around the tibial plateau. We observed no aspetic loosening, infection, or patella stress fracture. Survivorship analysis in the worst-case scenario showed a 90.9% success rate at 8 years. Copyright 2002, Elsevier Science (USA). All rights reserved.

Patella resurfacing in total knee replacement: functional evaluation and complications

Three different knee replacements, with three trochlear designs, were prospectively evaluated clinically and radiographically for patellar function and presence of patellar complications. They included the Insall-Burstein (IB) I and the IB-II (posterior cruciate ligament substituting) and the Meniscal Bearing Knee (MBK; posterior cruciate ligament recession). The trochlea of the IB-I was short and shallow with an anterior sharp edge of the intercondylar box (later modified to a smoother edge) and the femoral component had a prominent "shoulder." In the IB-II the trochlea was deeper to allow for soft tissue clearance. In the MBK the trochlea was more prolonged, with R and L components and the "shoulder" was less prominent. In all the cases the patella was resurfaced with an all polyethylene dome prosthesis. Knees with tibiofemoral problems were excluded. From the data of the present study the following conclusions can be drawn: (a) The most frequent problem was impingement (clunks) with the early version of the IB-I. Smoothening of the anterior edge significantly reduced the incidence of clunks to 5% in the modified IB I. (b) With the IB-II deepening the trochlea for soft tissue clearance improved the degree, not the incidence of clunks (4.5%), compared to the modified IB I. (c) With the MBK clunks were very rare and patellar function improved. (d) Throughout the three series patellar stress fractures and instability were rare and loosening or wear not evident. (e) Normal function (including stairs ascending and descending) can be expected in over 80% of category A patients. (f) Of the various radiological parameters only patella baja was correlated with symptoms in the IB prostheses. (g) We still prefer the dome design because is more tolerant and with cold flow may better conform to the trochlea increasing contact area.