Fixation Of Patellar Fractures Research Articles

Distinct risk profiles for short-term surgical complications and venous thromboembolism exist among extensor mechanism repair procedures.

Extensor mechanism injuries, which comprise patella fractures, patella tendon tearsand quadriceps tendon tears, are severely debilitating injuries and a common cause of traumatic knee pathology that requires surgical intervention. Risk factors for short-term surgical complications and venous thromboembolism (VTE) in this population have not been well characterised. The aim of this study was to identify perioperative risk factors associated with these short-term complications. The National Surgical Quality Improvement Program database was used to identify patients who underwent an isolated, primary extensor mechanism repair from 2015 to 2020. Patients were stratified by injury type. Demographic data were collected and compared. A multivariate logistic regression was used to control for demographic and comorbid factors while assessing risk factors for developing short-term complications. A total of 8355 patients were identified for inclusion in this study. Overall, 3% of patients sustained short-term surgical complications and 1% were diagnosed with VTE within 30 days of surgery. Patella fracture fixation had a nearly twofold higher risk for surgical complications compared to quadriceps tendon repair (p = 0.004). Patella tendon repair had a twofold higher risk for VTE (p = 0.045), specifically deep vein thrombosis (p = 0.020), compared to patella fracture fixation. Increasing age, smokingand American Society of Anesthesiologists Classifications 3 and 4 were also found to be risk factors for surgical complications (p = 0.012, p = 0.004, p = 0.011 and p = 0.032, respectively). This study used a nationally representative, widely validated, peer-reviewed database to provide valuable insights into risk factors for short-term postoperative complications associated with extensor mechanism repair procedures, revealing notable differences in risk profiles among distinct surgical procedures. The results of this study will inform surgeons and patients in enhancing risk assessment, guiding procedure-specific decision-making, optimising preoperative care, improving postoperative monitoringand contributing to future research of extensor mechanism injuries. Level III.

Transverse patella fracture fixation: A cadaveric biomechanical comparison of cannulated screws and anterior tension band versus low-profile, multiplanar mesh plating

IntroductionMultiplanar mesh plating of patella fractures has become more popular in recent years. It was the goal of this study to compare the biomechanical stability of cannulated screw with anterior tension band to multiplanar mesh plating for fixation of transverse patella fractures in cadaver specimens. Materials and MethodsEight matched pairs of fresh frozen cadaveric knees were obtained and soft tissues dissected leaving the extensor mechanism, joint capsule, and retinaculum intact. Transverse fractures were created at the mid-portion of the patella. For each pair, one specimen was repaired using cannulated screws with anterior tension band, and the second was repaired using multiplanar mesh plating. Each specimen underwent cyclic extension loading with loads increasing by 1.1 kg after every 50 cycles. Interfragmentary displacement was measured at the end of each interval at both 5° and 45° of knee flexion angle, with fixation failure defined by >2 mm displacement. ResultsThe specimens fixed with multiplanar mesh plating survived more cycles and higher loads than the specimens fixed with cannulated screws with anterior tension band (p = 0.011 comparing survival plots). After 150 cycles of extension loading, 3 of 8 of the specimens fixed with screws/tension band had failed, whereas none of the mesh plated specimens had failed. After 400 cycles, 7 of 8 of the screws/tension band had failed, whereas half of the mesh plated specimens had failed. ConclusionsWhile a more technically challenging and expensive technique, mesh plating for patella fractures appears to offer greater durability than traditional cannulated screw with tension banding.

Migration of patella tension band metal wires to popliteal fossa: Retrieval by posterior knee arthroscopy – A case report and literature review

Breakage of metal wires after fixation of patella fractures are common. However, migration of the broken wires to intraarticular location, popliteal fossa and distant sites can be least expected and a difficult scenario for retrieval. We describe in a 58-year male who presented with recurrent locking and migration of broken wires in popliteal fossa. he had fixation of patella fracture nine years ago. The metal wire was retrieved safely from the posterior lateral aspect of knee popliteal fossa by posterior knee arthroscopy using two posterior lateral portals. Through this case, we review the presentation of migrated wires, methods of retrieval and role of arthroscopy. Retrieval of migrated wires by knee arthroscopy can be successful in intraarticular location and in select cases at popliteal fossa, however with significant challenges. To avoid concerns with migration of wires, patients treated with wires for patella fractures should be adequately followed with radiography and sensitized for removal before breakage or at the time of breakage.

Lateral rim variable angle locked plating versus tension band wiring of simple and complex patella fractures: a biomechanical study.

Treatment of both simple and complex patella fractures is a challenging clinical problem. Although tension band wiring has been the standard of care, it can be associated with high complication rates. The aim of this study was to investigate the biomechanical performance of recently developed lateral rim variable angle locking plates versus tension band wiring used for fixation of simple and complex patella fractures. Sixteen pairs of human anatomical knees were used to simulate either two-part transverse simple AO/OTA 34-C1 or five-part complex AO/OTA 34-C3 patella fractures by means of osteotomies, with each fracture model created in eight pairs. The complex fracture pattern was characterized by a medial and a lateral proximal fragment, together with an inferomedial, an inferolateral, and an inferior (central distal) fragment mimicking comminution around the distal patellar pole. The specimens with simple fractures were pairwise assigned for fixation with either tension band wiring through two parallel cannulated screws or a lateral rim variable angle locking plate. The knees with complex fractures were pairwise treated with either tension band wiring through two parallel cannulated screws plus circumferential cerclage wiring or a lateral rim variable angle locking plate. Each specimen was tested over 5000 cycles by pulling on the quadriceps tendon, simulating active knee extension and passive knee flexion within the range of 90° flexion to full extension. Interfragmentary movements were captured via motion tracking. For both fracture types, the articular displacements measured between the proximal and distal fragments at the central patella aspect between 1000 and 5000 cycles, together with the relative rotations of these fragments around the mediolateral axis were all significantly smaller following the lateral rim variable angle locked plating compared with tension band wiring, p ≤ 0.01. From a biomechanical perspective, lateral rim variable angle locked plating of both simple and complex patella fractures provides superior construct stability versus tension band wiring under dynamic loading.

Incidence of postoperative complications in patellar fractures related to different methods of osteosynthesis procedures - a retrospective cohort study

BackgroundPatellar fractures have a comparatively low incidence compared to all fracture frequencies of the musculoskeletal system. However, surgical management is crucial to prevent postoperative complications that affect the knee joint. The purpose of the present study was to evaluate the incidence of postoperative complications and onset of postoperative osteoarthritis related to the chosen technique of patellar fracture management.MethodsIn a retrospective cohort study consecutive managed, isolated patella fractures were reviewed for demographic data, trauma mechanism, patella fracture type, fixation technique and postoperative complications. The results were documented radiographically and clinically and analysed statistically. The reporting followed the STROBE guidelines.ResultsA total of 112 patients were eligible for data evaluation. Surgical management of comminuted patellar fractures with small fragment screws showed significant fewer postoperative complications compared to other fixation techniques (8%, p < 0.043). The incidence of posttraumatic infection was significantly higher following the hybrid fixation technique with cannulated screws and tension wire than following the other analysed techniques (p = 0.024). No postoperative wound infection was observed after screw fixation or locking plate fixation. Symptomatic hardware was most frequently seen after tension-band fixation. Onset of posttraumatic osteoarthritis was most often found after the hybrid fixation technique (55%).ConclusionSurgical management of patellar fractures remains crucial but fracture fixation using plating systems or small fragment screws is least associated with postoperative complications.Trial registrationTrial registration number (DRKS):00027894.

PATELLA FRACTURE FIXATION WITH NOVEL WAGON WHEEL CONSTRUCT VERSUS TENSION-BAND CONSTRUCT: A TECHNICAL TRICK.

Internal fixation of patella fractures remains technically challenging. Cannulated screws with an anterior tension band have been associated with high rates of implant prominence, and fracture comminution can make appropriate application of a tension band impractical. We present the results of a novel technique using a transtendinous/transligamentous mini-fragment plate positioned peripherally around the patella with radially directed screws: termed the wagon-wheel (WW) construct. Compared with a cohort of fractures treated with cannulated screws with an anterior tension band, there was no difference in final range of motion and rate of nonunion. The WW construct had a significantly decreased incidence of symptomatic implants (5% vs. 32%, P = 0.02), rate of reoperation (9% vs. 38%, P = 0.018), dependency on gait aids (10% vs. 38%, P = 0.031), and a faster time to union (HR: 2.2; 95% CI, 1.28-3.95, P = 0.005). In summary, the WW was designed with the goal of obtaining peripheral plate fixation to maximize fragment-specific fixation while minimizing implant prominence. Patients treated with the WW demonstrated reduced rates of implant prominence and reoperation.

Fixation of patella fracture with non-metallic implant

Introduction Patella fracture fixations were traditionally done using metal tension band wiring techniques. However, complications associated with metallic implants causing soft tissue irritation are not infrequent. Since 1998, there have been emerging case reports and series reporting the use of non-metallic implants, such as braided sutures, in the fixation of patella fractures 3. However, there is a limited clinical study on a direct comparison between metal and suture fixation in patella fractures. The aim of this study is to evaluate the outcome of all-suture fixation in patella fractures and to compare all-suture and metal fixation in mid-pole fractures. Materials and method A total number of 50 patients with operated fracture patella were retrospectively reviewed in a 2.5-year period. Primary outcomes included operation time, time for fracture union and postoperative knee function. Secondary outcomes include complications, need for secondary operations due to prominent implants, infection or loss of fixation. Results There was no statistically significant difference concerning time for fracture union, operation time and postoperative functional knee score between the three fixation methods. The rate of secondary operations was higher in the hybrid group (7 out of 11, 63.6%) and metal group (7 out of 13, 53.8%), and was lowest in an all-suture group (0 out of 28, 0%). Metal implant breakage and soft tissue irritation were the main reasons for secondary operations, which was significantly lower in the all-suture group ( p &lt; 0.0003). Subgroup analysis for mid-pole patella fractures showed similar results in which all primary outcomes showed no statistically significant difference, while the need for secondary operations was higher in the metal group ( p &lt; 0.003). Conclusion All-suture fixation for patella fractures is a feasible option not limited to distal pole fractures. In our series, this technique achieved similar outcomes when compared with traditional metal implants, with lower rates of secondary operations, and could be safely employed in mid-pole fractures as well.

Response to “Letter to the editor on “Fixation of patella fractures with metallic implants is associated with a significantly higher risk of complications and re-operations than non-metallic implants: a systematic review and meta-analysis””

Response to “Letter to the editor on “Fixation of patella fractures with metallic implants is associated with a significantly higher risk of complications and re-operations than non-metallic implants: a systematic review and meta-analysis””

Biomechanical evaluation of an anatomical bone plate assembly for thin patella fracture fixation fabricated by titanium alloy 3D printing

This study established and evaluated the feasibility of a three-dimensional (3D)- printed titanium anatomical surface with adjustable thin bone plate assembly (AATBP) for patella fracture fixation. The AATBP was 1.6 mm in thickness and divided into a proximal plate (PP) with locking screw holes and a distal plate (DP) (0.4 mm in thickness) with compressive screw holes for assembly using a ratchet mechanism to adjust the total fixation height according to the patella size. Two pairs of hooks were designed on the proximal/distal edges to allow passage through the tendon to grip the fractured fragments. 3D printing combined with Computer Numerical Control (CNC) drilling was performed to manufacture the AATBP. Four-point bending and surface roughness tests were performed to evaluate the AATBP mechanical behavior. A cyclic (300 times) load test with 15-kg weights was adopted to compare the biomechanical stability between the AATBP and conventional tension band wiring (TBW) fixations. A parallel finite element (FE) analysis was achieved to understand the fracture gap and bone stress in the two different fixations on a transverse patella fracture. The result showed that the maximum AATBP manufacturing error was 3.75%. The average fracture gaps on the medial/lateral sides after cyclic loads were 2.38 &amp;plusmn; 0.57 mm/2.30 &amp;plusmn; 0.30 mm for TBW and 0.03 &amp;plusmn; 0.01 mm/0.06 &amp;plusmn; 0.03 mm for AATBP fixations. The same trend occurred in the FE simulation. This study confirmed that a complicated thin bone plate, including the anatomical surface, hooks, and ratchet with size-adjustable characteristics, can be fabricated using metal 3D printing with acceptable manufacturing error and reasonable anatomical surface/ thin bone plate assembly fitness. Biomechanical cyclic tests and FE simulation showed that the AATBP fixation is superior to the conventional TBW for patella transverse fractures.

A prospective study to estimate the outcome of patellar fracture fixation with suture material of high resistance properties

Background: Patella fractures make from 0.5% to 1.5% of all bone fractures. The most common fracture type, transverse fracture, impairs knee extension mechanics due to fragment displacement. In the years that followed, newer procedures were pursued in order to alleviate the aforementioned difficulties. K-wires were successfully replaced with cannulated screws. Furthermore, cerclage wiring was replaced with alternative implants without jeopardising the figure-of-eight tension band build. Methods: A prospective study was conducted over a period of 18 months to evaluate the outcome of patellar fractures fixation with newer techniques and sutures. The study had 35 participants. Results: This study included patients in the age group of 21-77 years. Among the total of 35 patients in the study, Maximum number of patients in our study were in the age range of 41-60 years which comprised 42.86%. In this study of 35 patients, 20 were male patients and 15 patients were female i.e. 57.14% of our study group comprised of male patients while 42.86% comprised of female patients. Conclusions: The above study clearly demonstrates that the high resistance suture materials can be used as a potential alternative or may even be better to the existing prevalent fixation of patella fractures with stainless steel and wires. Fibertape fixation presents a lot of advantages over the traditional stainless steel and wire fixation. Biomechanically, fibertape has demonstrated tensile strength and stiffness equal to stainless steel and in certain specific parameters proved even more stronger than the stainless steel.

Predictors of postoperative complications of tension band wiring techniques for patella fracture: A retrospective multicenter (TRON group) study

ObjectiveThe relationship between postoperative complications and operative techniques of tension band wiring (TBW) is not well studied. We aimed to evaluate the incidence of implant breakage, implant migration, and loss of reduction in patellar fractures treated with TBW and identify radiographic factors associated with these postoperative complications. MethodsThis multicenter (named, TRON group) retrospective study included 224 patients who underwent open reduction and internal fixation of patella fractures using TBW from January 2016 to December 2020. Radiographic findings were evaluated by experienced orthopedic surgeons, and radiographic outcomes were assessed for K-wire migration, implant breakage, and loss of reduction. Logistic regression analysis was performed to identify radiographic factors associated with postoperative complications. ResultsImplant migration occurred in 44 cases (19.6%), with bending of a single K-wire end identified as a significant risk factor (OR: 12.90; 95% CI: 4.99–33.30; P < 0.001). Implant breakage occurred in 43 cases (19.2%), with a large patella-tension band ratio (OR: 291.0; 95% CI: 19.60–4330; P < 0.001) and a wide distance between K-wires (OR: 1.15; 95% CI: 1.060–1.250; P = 0.001) identified as significant risk factors. Loss of reduction occurred in 5 cases (2.0%), but no significant risk factors were identified. ConclusionThis study highlights the importance of bending both ends of the K-wires and proper placement of the tension band and K-wires in reducing postoperative complications in patellar fractures treated with TBW. Further research is needed to better understand the risk factors associated with loss of reduction.

Adjunct neutralization plating in patella fracture fixation: a technical trick-Erratum.

Adjunct neutralization plating in patella fracture fixation: a technical trick-Erratum.

Biomechanical Evaluation of Transverse Patella Fracture Fixation using Headless Screws: A Comparison of Suture vs. Cable as a Tension Band.

To explore the performance of headless screws with FiberWire Suture as a tension band and headless screws with a mini-cable tension band in patella fixation. A transverse osteotomy was created in six matched pairs of fresh-frozen cadaver knee joints. One knee was randomly assigned to receive fixation with headless screws plus a cable tension band (Synthes 1.3 mm cable with crimp), while the other was fixed with headless screws plus a suture tension band (#5 FiberWire). Using a servo-hydraulic material testing system, the specimens were first tested non-destructively under 20% of the reported mean failure load with standard technique of cannulated screws with tension band wiring. The specimen was then loaded to 1000 N to test the construct's failure strength. All tests were run under displacement-control with loading threshold. A motion analysis system was used to track the interfragmentary motion to assess fixation stability. The correlation between fixation strength and score of bone density was also examined. In the non-destructive loading test, gap displacement under 150N was 0.10 mm or less for 11 of 12 specimens, and the difference between the two groups was not statistically significant. In the destructive test, 3 of 12 specimens maintained reduction (gap<2mm) at the maximum load of 1000 N. Of the failed specimens, the mean strength was 648±185 N for suture and 784±228 N for cable (p=0.24). Regression analysis shows bone density was associated with the failure load of the tendon (R2=0.43, p=0.02). There was no significant difference in fixation strength or sub-failure fragment displacement between the suture and cable tension band techniques when using headless screws, but suture on average was weaker.

A novel patella fracture fixation technique: finite element analysis.

Patella fractures account for approximately 1% of all bone fractures. The tension band wiring technique has been used in surgical treatment. However, there is no clear information about the location of the K-wires in sagittal plane. Thus, a transverse fracture line was created in the patella finite element model and fixed with Kirchner (k) wires and cerclage at different angles and compared with two different standard tension band models. A total of 10 finite element models were created to study AO/OTA 34-C1 patella fractures. Two models used the classical tension band method with either circumferential or 8-shaped cerclage wire. The other 8 models used K-wires placed at 45° or 60°, either alone or combination with cerclage wire. A force of 200N, 400N, and 800N were applied at 45° knee angle and the resulting data fracture line opening, surface pressure and stress in the implants were analyzed through finite element analysis. When all the results are considered, it was determined that the K-wires 60° crossing at the fracture line and with cerclage modeling was superior to the other models. The diagonal placement of the K-wires with cerclage (could be 45° or 60° medium) was superior to the reference models. This study has shown that the new fixation method we propose could come to the fore as an alternative method to be used successfully in transverse patella fractures and lower complications. In transverse patellar fractures, the use of K-wires crossed at 60° may be a good alternative to the standard method.

Achieving Rapid Healing and Low Complication Rates in Patellar Fracture Fixation: The Benefits of Cerclage and Figure-of-Eight Configuration.

Background and objective Patellarfractures account for around 1% of all fractures. Conservative treatment is advised in patients without any incompatibility of articular surfaces or those with intact extensor mechanisms. More than a 2-mm articular gap due to fracture warrants surgical intervention. Tension band wiring (TBW) is a commonly used practice for fixation, However, there is still controversy about its effectiveness and complications arising due to the hardware. Modification of this technique by using K-wires has been considered a method of choice, but this technique is associated with complications due to K-wires. The Pyrford technique is a method for patellar fracture fixation by circumferential cerclageand anterior TBW. We employed the figure-of-eight configuration over the circumferential wire. This study aimed to analyze the outcomes ofTBW of the patella without K-wires by assessing the rate of complication and functional outcomes. Materials and methods A total of 38 patientswithOTA 34C type, simple and comminuted type of patella fractures aged between22 and 70 years were treated with circumferential cerclage and figure-of-eight TBW.All patients underwent patellar fixation with cerclage and through direct purchase of SS wire via quadriceps and patellar tendon. Patients were followed up for one to three years. We analyzed differences in the range of motion, fracture reduction, fracture healing time, Bostman score for knee function, and complications. Results The mean age of the patients was 45 years. After TBW without K-wires, fracture healing and functional outcomes were satisfactory according to patient feedback and clinocoradiological examinations. Of note, 35 out of 38 patients (92%) had gained up to 90 degrees of active flexion at the end of one week. One patient (2.42%) developed a superficial infection. All fractures had achieved union at the end of 16 weeks. Malunion or nonunion was not noted in any of the cases. There was no case of implant removal. The average Bostman score at the 12-month follow-up was 28.5 ±1.5. The incidence of complications due to K-wire was nullified. Conclusion Based on our findings, the described method leads to better functional outcomes, decreasing hardware-related complications, and can be used in simple as well as comminuted fractures. The fracture healing and functional outcomes and rate of complications were satisfactory.

A Double Cerclage and Tension band Wiring Technique Fixation for Comminuted Patellar Fractures

Background: When the patella is fractured it should be repaired and patellectomy should be avoided as much as possible even in comminuted fractures. In comminuted patellar fractures, a combination of cerclage wiring and tension band fixation is confirmed to provide good mechanical stability. The objective of the present work was to find out the effectiveness of comminuted patellar fracture fixation with combined cerclage and tension band wiring technique. Method and patient: Forty- six patients underwent surgical stabilization for comminuted patellar fracture between year 1996 to 2016. All fractures were classified as 34-C3 according to the Orthopedic Trauma Association Classification. All the patients were underwent open reduction- internal fixation with combined Cerclage and tension band wiring technique. Result: Fracture healing was occurred within an average of 3- 5 months, while in bilateral cases it was 4 - 8 months. Full range of knee movements was seen in 42 (91.3 %) cases and knee flexion more than 120º was seen in 4 (8.7 %) cases. Quadriceps power was normal in 44 (95.6 %) cases, but 2 cases had quadriceps power of grade-IV. No extensor lag and no quadriceps atrophy were seen in any cases. Forty four (95.6 %) patients could squat fully and two (4.3 %) patients could squat up to half of the normal level. All patients were able to climb up and down stairs without any problem. Most patients have no subjective complaints like pain, difficulty in walking, difficulty in squatting and climbing. The clinical results after operation were evaluated by the clinical grading scales of Böstman including range of movement, pain, work, atrophy, assistance in walking, effusion, giving way, and stair-climbing during follow-up. There were no significant complications, such as infection, knee stiffness, and non-union or implant failure in any of the cases. The average follow-up period was 8 months (Ranged 6 – 18 months). Conclusions: This study had concluded that combined Cerclage and tension band wiring technique was a successful surgical treatment for comminuted fractures of the Patella.

Non-metallic fixation of transverse patella fracture using non-absorbable polyester suture

This study aims to evaluate the functional outcomes for the fixation of transverse patella fracture (TPF) using a transosseous suturing method with a non-absorbable polyester suture. A retrospective case series method was conducted on 37 patients who underwent open reduction and internal fixation using a transosseous suturing technique with a non-absorbable polyester suture at Lower Limb Department, Hospital for Traumatology and Orthopaedics, Ho Chi Minh City, from 1/6/2020 to 1/6/2022. Results showed that the surgery time was 37.43±9.76 minutes; average follow-up time was 8.57±5.35 months. The results of facet joint after surgery and bone healing were 100% (very good and good); bone healing time was 14.22±2.78 weeks; rehabilitation time was 13.84±3.3 weeks; knee function according to Lysholm scale achieved very good and good results with the average score of 97.62±3.7 points; no hardware-related complications in the study. One patient fell again and broke the kneecap; 100% of the patients were satisfied after the treatment, especially with the reason that there was no need for a second surgery to get the instruments. In conclusion, the fixation of TPF using a transosseous suturing method with a non-absorbable polyester suture is safe and effective with no hardware-related complications and does not require a second surgery to instrument collection during the mean follow-up period of 8.57±5.35 months.

Modified tension band wiring of transverse patella fractures through cannulated cancellous screws: An analysis of functional outcomes and complications

Introduction: The management of transverse patella fractures with modified tension band wiring using Kirschner wires, though routinely used is associated with complications such as implant loosening, wire migration, and infection. We hypothesized that using cannulated cancellous screws with tension band wiring through the screws will overcome these complications. Objective: The objective was to assess the functional and radiological outcomes following patellar fracture fixation with modified tension band wiring through cannulated cancellous screws. Materials and Methods: Twenty-four patients with transverse patella fractures fixed with modified tension band wiring through cannulated cancellous screws were included in this retrospective study. Surgical technique involved a vertical incision over the knee, reduction of the fracture initial fixation with Kirschner wires, replaced with cannulated cancellous screws. A stainless steel wire was passed through the lumen of the screws, and tension band was applied. Follow-up at 3, 6, and 12 months was done to assess the radiological union. Functional outcome was assessed with the Bostman scoring. Results: The study included 13 male and 11 female patients with an average age of 39.6 years, with 8 three-part and 16 two-part fractures. The average follow-up was 14 months, and the average time to union was 8.5 weeks. Twelve patients had an excellent, 11 patients a good, and one patient an unsatisfactory functional outcome as per the Bostman score. No infection or implant loosening was observed. Conclusion: The technique of patella fracture fixation with modified tension band through cannulated cancellous screws offers an enhanced stability, minimizing the complications such as implant migration, prominent implant, and refracture. As such, we recommend this technique in all transverse patella fractures.

Patellar fracture fixation using cannulated screws and Fiber Wire tension band

Patellar fracture fixation using cannulated screws and Fiber Wire tension band

Clinical efficacy research of arthroscopy-assisted closed reduction-guided internal fixation of patella fractures

Clinical efficacy research of arthroscopy-assisted closed reduction-guided internal fixation of patella fractures