Depth of subchondral perforation influences the outcome of bone marrow stimulation cartilage repair

Subchondral drilling and microfracture are bone marrow stimulation techniques commonly used for the treatment of cartilage defects. Few studies to date have examined the technical variants which may influence the success of the cartilage repair procedures. This study compared the effect of hole depth (6 mm vs. 2 mm) and hole type (drill vs. microfracture) on chondral defect repair using a mature rabbit model. Results from quantitative histomorphometry and histological scoring showed that deeper versus shallower drilling elicited a greater fill of the cartilage defect with a more hyaline character in the repair matrix indicated by significant improvement (p = 0.021) in the aggregate measure of increased cartilage defect fill, increased glycosaminoglycan and type II collagen content and reduced type I collagen content of total soft repair tissue. Compared to microfracture at the same 2 mm depth, drilling to 2 mm produced a similar quantity and quality of cartilage repair (p = 0.120) according to the aggregate indicator described above. We conclude that the depth of bone marrow stimulation can exert important influences on cartilage repair outcomes. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 29: 1178–1184, 2011     

Drilling and microfracture lead to different bone structure and necrosis during bone‐marrow stimulation for cartilage repair

Bone marrow stimulation is performed using several surgical techniques that have not been systematically compared or optimized for a desired cartilage repair outcome. In this study, we investigated acute osteochondral characteristics following microfracture and comparing to drilling in a mature rabbit model of cartilage repair. Microfracture holes were made to a depth of 2 mm and drill holes to either 2 mm or 6 mm under cooled irrigation. Animals were sacrificed 1 day postoperatively and subchondral bone assessed by histology and micro‐CT. We confirmed one hypothesis that microfracture produces fractured and compacted bone around holes, essentially sealing them off from viable bone marrow and potentially impeding repair. In contrast, drilling cleanly removed bone from the holes to provide access channels to marrow stroma. Our second hypothesis that drilling would cause greater osteocyte death than microfracture due to heat necrosis was not substantiated, because more empty osteocyte lacunae were associated with microfracture than drilling, probably due to shearing and crushing of adjacent bone. Drilling deeper to 6 mm versus 2 mm penetrated the epiphyseal scar in this model and led to greater subchondral hematoma. Our study revealed distinct differences between microfracture and drilling for acute subchondral bone structure and osteocyte necrosis. Additional ongoing studies suggest these differences significantly affect long‐term cartilage repair outcome. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:1432–1438, 2009     

Pirfenidone reduces subchondral bone loss and fibrosis after murine knee cartilage injury

Pirfenidone is an anti‐inflammatory and anti‐fibrotic drug that has shown efficacy in lung and kidney fibrosis. Because inflammation and fibrosis have been linked to the progression of osteoarthritis, we investigated the effects of oral Pirfenidone in a mouse model of cartilage injury, which results in chronic inflammation and joint‐wide fibrosis in mice that lack hyaluronan synthase 1 (Has1^?/?) in comparison to wild‐type. Femoral cartilage was surgically injured in wild‐type and Has1^?/? mice, and Pirfenidone was administered in food starting after 3 days. At 4 weeks, Pirfenidone reduced the appearance, on micro‐computed tomography, of pitting in subchondral bone at, and cortical bone surrounding, the site of cartilage injury. This corresponded with a reduction in fibrotic tissue deposits as observed with gross joint surface photography. Pirfenidone resulted in significant recovery of trabecular bone parameters affected by joint injury in Has1^?/? mice, although the effect in wild‐type was less pronounced. Pirfenidone also increased Safranin‐O staining of growth plate cartilage after cartilage injury and sham operation in both genotypes. Taken together with the expression of selected extracellular matrix, inflammation, and fibrosis genes, these results indicate that Pirfenidone may confer chondrogenic and bone‐protective effects, although the well‐known anti‐fibrotic effects of Pirfenidone may occur earlier in the wound‐healing response than the time point examined in this study. Further investigations to identify the specific cell populations in the joint and signaling pathways that are responsive to Pirfenidone are warranted, as Pirfenidone and other anti‐fibrotic drugs may encourage tissue repair and prevent progression of post‐traumatic osteoarthritis. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:365–376, 2018.

     

降钙素在体内体外实验中对兔关节炎关节软骨退变及软骨下骨骨代谢的影响

[目的]研究降钙素(calcitonin, CT)对骨性关节炎关节软骨退变和软骨下骨骨代谢的影响.[方法]30只3个月龄雌性日本大耳白兔随机分为三组,其中两组行右膝关节前交叉韧带切断术(anterior cruciate ligament transaction,ACLT),分为ACLT+CT组和ACLT+NS组,第3组为Sham组.ACLT+CT给予每日1次皮下注射降钙素5 IU/(kg·d),持续8周,ACLT+NS组给予同样剂量生理盐水.术后8周后处死所有动物.取股骨髁制成切片行MMP-13和Ⅱ型胶原免疫组化染色.取胫骨近端制成硬组织切片行骨形态计量学检测.体外实验中,取兔膝关节软骨,经消化、培养,将第3代软骨细胞分三组:向IL-1β组加入人重组IL-1β(10 ng/ml). IL-1β+CT组加入人重组IL-1β (10 ng/ml)2 d后,再向培养液中加入CT(50 ng/ml).正常组不加任何诱导剂和干扰剂培养.然后行MMP-13、Ⅱ型胶原免疫组化检测和Realtime RT-PCR法检测.[结果]Sham组和ACLT+CT组软骨下骨骨小梁相对体积和厚度等均显著高于ACLT+NS组.Sham组和ACLT+CT组的Ⅱ型胶原的光密度值均显著高于ACLT+NS组,而MMP-13的光密度值显著低于ACLT+NS组(P<0.05).正常组和IL-1β+CT组的Ⅱ型胶原光密度值均显著高于IL-1β组而MMP-13的光密度值都显著低于IL-1β组(P<0.05).在正常组和IL-1β+CT组中Ⅱ型胶原的mRNA含量均显著高于IL-1β组而MMP-13的mRNA含量均显著低于IL-1β组(P<0.05).[结论]降钙素5 IU/(kg·d)皮下注射能够增加ACLT兔膝关节软骨Ⅱ型胶原的分泌和抑制MMP-13的表达,并可能通过调节软骨下骨的骨代谢和微结构来保护关节软骨; CT(50 ng/ml)能增加体外培养的含有IL-1β(10 ng/ml)的软骨细胞中Ⅱ型胶原的含量和抑制MMP-13分泌.     

Similarities and discrepancies in subchondral bone structure in two differently induced canine models of osteoarthritis

In osteoarthritis (OA), cartilage degradation is accompanied by subchondral bone changes. The pathogenesis and physiology of bone changes in OA are still unclear. The changes in subchondral bone architecture and cartilage damage were compared in differently induced experimental models of OA. Experimental OA was induced bilaterally by anterior cruciate ligament transection (ACLT) or by cartilage trauma (Groove model); bilateral sham surgery served as control. Lysylpyridinoline (LP, bone resorption) and C‐telopeptide of type II collagen (CTX‐II, cartilage breakdown) were measured over time. At 20 weeks after surgery, the subchondral cortical plate and trabecular bone of the tibia were analyzed by micro–computed tomography (µCT) and cartilage degeneration was analyzed histologically and biochemically. In both models, cartilage degeneration and cortical subchondral plate thinning were present. CTX‐II levels were elevated over time in both models. Subchondral trabecular bone changes were observed only in the ACLT model, not in the Groove model. Correspondingly, LP levels were elevated over time in the ACLT model and not in the Groove model. Interestingly, the trabecular bone changes in the ACLT model were extended to the metaphyseal area. The early decrease in plate thickness, present in both models, as was cartilage damage, suggests that plate thinning is a phenomenon that is intrinsic to the process of OA independent of the cause/induction of OA. On the other hand, trabecular changes in subchondral and metaphyseal bone are not part of a common pathway of OA development and may be induced biomechanically in the destabilized and less loaded ACLT joint. © 2010 American Society for Bone and Mineral Research     

Formation of cartilage repair tissue in articular cartilage defects pretreated with microfracture and covered with cell‐free polymer‐based implants

The aim of our study was to evaluate the mid‐term outcome of a cell‐free polymer‐based cartilage repair approach in a sheep cartilage defect model in comparison to microfracture treatment. Cell‐free, freeze‐dried implants (chondrotissue®) made of a poly‐glycolic acid (PGA) scaffold and hyaluronan were immersed in autologous serum and used for covering microfractured full‐thickness articular cartilage defects of the sheep (n = 4). Defects treated with microfracture only served as controls (n = 4). Six months after implantation, cartilage implants and controls were analyzed by immunohistochemical staining of type II collagen, histological staining of proteoglycans, and histological scoring. Histological analysis showed the formation of a cartilaginous repair tissue rich in proteoglycans. Histological scoring documented significant improvement of repair tissue formation when the defects were covered with the cell‐free implant, compared to controls treated with microfracture. Immunohistochemistry showed that the cell‐free implant induced cartilaginous repair tissue and type II collagen. Controls treated with microfracture showed marginal formation of a mixed‐type repair tissue consisting of cartilaginous tissue and fibro‐cartilage. Covering of microfractured defects with the cell‐free polymer‐based cartilage implant is suggested to be a promising treatment option for cartilage defects and improves the regeneration of articular cartilage. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:1353–1360, 2009     

Histomorphochemical comparison of microfracture as a first‐line and a salvage procedure: Is microfracture still a viable option for knee cartilage repair in a salvage situation?

Microfracture is considered as the first‐line procedure for knee cartilage repair, but the results of microfracture seem less predictable and rather controversial in a salvage situation. Thus, the purpose of the study was to histomorphochemically compare microfracture as a salvage procedure with microfracture as a first‐line procedure in a rabbit model. We hypothesized that microfracture in a salvage situation would result in histomorphochemically inferior cartilage repair compared to microfracture as a first‐line procedure, and the inferiority would be attributed to less migration of reparable marrow cells to the defect due to destruction of microarchitecture of the subchondral bone. Thirty‐six New Zealand white rabbits were divided into three groups: (i) untreated full‐thickness chondral defect, (ii) single microfracture treatment (first microfracture group), and (iii) repeated microfracture in 8 weeks after the first procedure (second microfracture group). In each group, rabbits were sacrificed at the end of 8 weeks, and osteochondral specimens at the repair sites were obtained for histomorphochemical analysis. Results showed that microfracture as a salvage procedure resulted in overall inferior cartilage repair histomorphochemically compared with microfracture as a first‐line procedure, which correlated with deteriorative changes in the quality of underlying subchondral bone rather than intrinsic incapability to recruit the reparative cells in the defect area. In conclusion, although a comparable number of reparable cells and a mechanically weakened subchondral bone are anticipated, more study is necessary to clearly determine when a microfracture should be performed in a situation. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:802–810, 2014.     

复合材料置入软骨下骨诱发兔膝骨关节炎的实验研究

[目的]通过增加软骨下骨硬度模拟软骨下骨硬化,诱发骨关节炎的发生,探讨骨关节炎发病机制.[方法]调整聚甲基丙烯酸甲酯粉剂(polymethylmethacrylate,PMMA)、甲基丙烯酸甲酯液剂(methylmethacrylate,MMA)、羟基磷灰石(hydroxyapatite,HA)和蒸馏水的比例,使反应温度低于40℃,测量该比例下聚合后的极限强度和刚度并与软骨下骨比较,制得PMMA/HA复合材料.刮除兔胫骨平台内侧软骨下骨后置入复合材料,对术后3、6、9、12周的关节软骨进行组织学观察,免疫组化法榆测Ⅱ型胶原和基质会属蛋白酶-1(matrix metallo proteinase-1,MMP-1)在软骨中的表达和分布,并与空白、对照组比较.透射电镜观察空白、6、12周组软骨细胞改变.[结果]该复合材料置入软骨下骨后,随观察时间延长实验组逐渐出现退变,Mankin分级逐渐升高,电镜也显示实验组软骨细胞退变表现.免疫组化显示Ⅱ型胶原表达增加主要在移行层和深层上部,MMP-1表达以软骨表层及中上层居多,随观察时间延长二者染色强度均逐渐升高.[结论]增加软骨下骨硬度后,诱发了兔膝骨关节炎.提示软骨下骨硬化可引起软骨退变,其可能是骨关节炎的病因之一.     

Subchondral bone fragility with meniscal tear accelerates and parathyroid hormone decelerates articular cartilage degeneration in rat osteoarthritis model

The aims of this study were to investigate the influence of subchondral bone fragility (SBF) on the progression of the knee osteoarthritis by using a novel rat model, and to examine the preventive effect of parathyroid hormone (PTH) on cartilage degeneration. First, 40 rats were assigned to the following four groups: Sham, SBF, Medial meniscal tear (MMT), and MMT + SBF groups. In SBF and MMT + SBF groups, we induced SBF by microdrilling the subchondral bone. Second, 10 additional rats were randomly assigned to the following two groups: MMT + SBF + saline and MMT + SBF + PTH groups. Osteoarthritic changes in the articular cartilage and subchondral bone were evaluated using safranin‐O/fast green staining, matrix metalloproteinase‐13 (MMP‐13), and type X collagen immunohistochemistry, toluidine blue staining, and micro‐CT scanning. The combination of SBF and meniscal tear increased the number of mast cells in the subchondral bone, and led to the abnormal subchondral bone microarchitecture, such as abnormally decreased trabecular number and increased trabecular thickness, compared with meniscal tear alone. Moreover, SBF with meniscal tear enhanced articular cartilage degeneration and increased the expression of MMP‐13 and type X collagen, compared with meniscal tear alone. The administration of PTH decreased the number of mast cells in the subchondral bone and improved the microstructural parameters of the subchondral bone, and delayed the progression of articular cartilage degeneration. These results suggest that SBF is one of the factors underlying the osteoarthritis development, especially in knees with traumatic osteoarthritis, and that the administration of PTH is a potential therapeutic treatment for preventing OA progression. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1959–1968, 2018.

     

膝关节原发性骨关节炎软骨和软骨下骨病理改变的定量研究

目的观察膝关节原发性骨关节炎(osteoarthritis,OA)胫骨平台软骨和软骨下骨病理改变特点,对比内、外侧平台软骨和软骨下骨结构参数,探讨钙化层和软骨下骨在OA发病机制中的作用。方法取2009年10月-2011年5月行人工全膝关节置换术治疗的30例30膝原发性OA患者自愿捐赠的新鲜胫骨平台标本进行实验。其中男11例,女19例;年龄55～78岁,平均65.1岁。病程10～25年,平均16.6年;患膝内翻畸形1～23°,平均9.3°。大体观察胫骨平台后在内、外侧中央负重区取材,常规制备脱钙石蜡切片,行HE和番红O/固绿染色,观察关节软骨退变特点,参照Mankin评分标准评分并分期;观察钙化层及软骨下骨病理改变。应用Image Pro Plus 6.0图像分析软件测量软骨和软骨下骨结构参数,包括软骨全层(total articular cartilage,TAC)厚度、钙化层(articular calcified cartilage,ACC)厚度、ACC/TAC比值、软骨下骨板(subchondral bone plate,SCP)厚度以及骨小梁体积分数(trabecular bone volume,BV/TV)。结果大体观察内侧平台软骨退变较外侧严重,内侧平台软骨Mankin评分为(12.4±1.1)分,显著高于外侧平台的(8.3±1.6)分(t=12.173,P=0.000)。根据Mankin评分结果在60个标本中,14个为OA早期,可见软骨浅表层裂隙、潮线复制和软骨下骨增厚;19个为OA中期,可见软骨深层裂隙、多发软骨下骨吸收陷窝和明显增厚的软骨下骨;27个为OA晚期,可见软骨全层缺失、软骨内化骨和"象牙化"软骨下骨。软骨和软骨下骨结构参数测定示:内侧平台TAC厚度显著低于外侧平台,ACC/TAC比值、BV/TV及SCP厚度显著高于外侧平台,差异均有统计学意义(P<0.05)。内、外侧平台ACC厚度比较,差异无统计学意义(P>0.05)。结论钙化层和软骨下骨可能在OA发生与进展中发挥了重要作用。     

Implantation of bone marrow-derived buffy coat can supplement bone marrow stimulation for articular cartilage repair

Objective

Bone marrow stimulation (BMS) has been regarded as a first line procedure for repair of articular cartilage. However, repaired cartilage from BMS is known to be unlike that of hyaline cartilage and its inner endurance is not guaranteed. The reason presumably came from a shortage of cartilage-forming cells in blood clots derived by BMS. In order to increase repairable cellularity, the feasibility of autologous bone marrow-derived buffy coat transplantation in repair of large full-thickness cartilage defects was investigated in this study.

Methods

Rabbits were divided into four groups: the defect remained untreated as a negative control; performance of BMS only (BMS group); BMS followed by supplementation of autologous bone marrow buffy coat (Buffy coat group); transplantation of autologous osteochondral transplantation (AOTS) as a positive control.

Results

Repair of cartilage defects in the Buffy coat group in a rabbit model was more effective than BMS alone and similar to AOTS. Gross findings, histological analysis, histological scoring, immunohistochemistry, and chemical assay demonstrated that supplementation of autologous bone marrow buffy coat after BMS arthroplasty effectively repaired cartilage defects in a rabbit model, and was more effective than BMS arthroplasty alone.

Conclusion

Supplementation of autologous bone marrow-derived buffy coat in cases of BMS could be a useful clinical protocol for cartilage repair.     

兔自体松质骨颗粒修复关节软骨损伤

目的研究兔自体松质骨颗粒在膝关节软骨损伤处移植后能够诱导软骨组织生成、促进关节软骨损伤修复的现象。方法 12只新西兰大白兔麻醉后在兔的右侧膝关节股骨远端内、外侧髁负重区用电钻分别造成直径、深度均为3 mm的骨软骨缺损创面,取同侧髂骨松质骨,制成直径约为1 mm松质骨颗粒植入股骨内侧髁软骨缺损处,作为实验组,外侧髁软骨缺损不做处理作为对照组。术后12周进行膝关节大体观察、病理切片染色,评估关节软骨损伤的修复情况。结果兔膝关节实验组软骨缺损处被新生软骨填充,软骨面光滑,组织切片染色显示有关节软骨形成。对照组缺损创面仍然凹陷,仅在缺损边缘有少量软骨生长。结论兔自体松质骨颗粒在膝关节软骨损伤处能够诱导软骨生成,促进关节软骨的修复,是一种良好的关节软骨损伤修复方法。     

Healing of articular cartilage defects. An experimental study of vascular and minimal vascular microenvironment.

The repair potential in a chondral defect with (treatment A) or without access to bone marrow elements (treatment B) at the basis of the defect sealed by a rim-sutured periosteal flap was studied using adult New Zealand rabbits (22 weeks) as an experimental model. At sacrifice, macroscopical changes, synovial fluid contents, degree of filling, thickness of the cartilage rim, and the subchondral bone were evaluated. Histomorphometric measurements of extent of filling (mainly fibrous tissue) of the defect at 36 weeks postoperatively, showed 50% filling in treatment A compared with 33% in treatment B (p = 0.011). A difference in height of the cartilage rim between the experimental groups and sham-control was measured ( p = 0.005). Cartilage degeneration was observed at the cartilage rim of the original defect, and included loss of chondrocytes and disruption of surface continuity in both experimental groups. In addition, treatment A resulted in a significantly increased thickness of the subchondral bone in the defect in comparison to treatment B at 2 weeks and at 36 weeks (p = 0.021). The increased thickness of the subchondral bone may be of concern for the bone marrow stimulation techniques regarding the long-term outcome.     

Injury and repair of articular cartilage: Related scientific issues

Articular cartilage (AC) is the soft tissue lining the ends of bones in diarthrodial joints. It is responsible for providinglubrication and compressive stiffness to the joint during articulation while responding viscoelastically to mechanical loading. Injury of the tissue caused by trauma or disease can be devastating to joint function as these mechanisms fail. Because of this, it is essential to review the basic science underlying the mechanical roles AC plays while healthy, the biomechanical and biological perspectives of the injury and repair processes, and the current repair techniques available for injured AC. This is done in an effort to further our understanding of the healing capacity of AC and facilitate new efforts into AC repair.     

Attenuation of cartilage degeneration by calcitonin gene‐related paptide receptor antagonist via inhibition of subchondral bone sclerosis in osteoarthritis mice

Osteoarthritis (OA) is a progressive joint disorder which affects cartilage and subchondral bone. Calcitonin gene‐related peptide (CGRP) plays a role in bone metabolism. The purpose of this study is to examine the therapeutic effect of the blocking CGRP on OA progression in mice by inhibition of subchondral bone sclerosis. OA was induced by the resection of the medial meniscotibial ligament of the knee in C57/BL6 mice. An intraperitoneal injection of the CGRP receptor antagonist (BIBN4096) was administered after OA surgery. At 1, 4, and 8 weeks after injection, histological analysis were performed. In vitro, the effect of CGRP and BIBN4096 on osteogenesis and osteoclastogenesis was analyzed. BIBN4096 could prevent cartilage degeneration and subchondral bone sclerosis. The OARSI score in the BIBN4096 group was significantly lower than that in the control. In vitro, CGRP up regulated osteocalcin expression, but its expression was down regulated by BIBN4096. CGRP inhibited osteoclastogenesis of raw 267.4 cells, but its effect was reduced by the addition of BIBN4096.The current study showed that subchondral bone sclerosis and increasing expression of CGRP occurs in the early phase of OA in relation to cartilage degeneration, and that BIBN4096 could effectively attenuate OA progression. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1177–1184, 2016.     

High fat diet accelerates cartilage repair in DBA/1 mice

Obesity is a well‐known risk factor for osteoarthritis, but it is unknown what it does on cartilage repair. Here we investigated whether a high fat diet (HFD) influences cartilage repair in a mouse model of cartilage repair. We fed DBA/1 mice control or HFD (60% energy from fat). After 2 weeks, a full thickness cartilage defect was made in the trochlear groove. Mice were sacrificed, 1, 8, and 24 weeks after operation. Cartilage repair was evaluated on histology. Serum glucose, insulin and amyloid A were measured 24 h before operation and at endpoints. Immunohistochemical staining was performed on synovium and adipose tissue to evaluate macrophage infiltration and phenotype. One week after operation, mice on HFD had defect filling with fibroblast‐like cells and more cartilage repair as indicated by a lower Pineda score. After 8 weeks, mice on a HFD still had a lower Pineda score. After 24 weeks, no mice had complete cartilage repair and we did not detect a significant difference in cartilage repair between diets. Bodyweight was increased by HFD, whereas serum glucose, amyloid A and insulin were not influenced. Macrophage infiltration and phenotype in adipose tissue and synovium were not influenced by HFD. In contrast to common wisdom, HFD accelerated intrinsic cartilage repair in DBA/1 mice on the short term. Resistance to HFD induced inflammatory and metabolic changes could be associated with accelerated cartilage repair. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1258–1264, 2017.

     

Demineralized allogeneic bone matrix for cartilage repair

We tested the chondrogenic potential of demineralized allogeneic bone matrix (DABM) in the repair of osteochondral defects. In 42 adult rabbits, a 5-mm2 or 15-mm2 defect was created bilaterally in the intercondylar groove of distal femur. DABM was inserted directly in 37 defects, whereas in 35 it was inserted after previous placement in muscle for 4, 16, or 19 days. Another 12 defects were left empty, serving as controls. Subgroups of animals were killed at 6, 12, 18, and 26 weeks. The distal femora were excised and prepared for histologic evaluation in hematoxylin-eosin and toluidine blue. Cartilage-like repair tissue was observed in the majority of defects. However, there was a great variability in the experimental groups without any clear relationship to type of DABM implant, defect size, or postoperative time. Even individual knees exhibited varying stages of cartilage differentiation. Overall, DABM placed in muscle for 19 days appeared to yield the best repair of the defects. The most consistent findings of the present study were bone formation in the marrow of distal femur and, notably, the absence of bone differentiation toward the joint surface. Hence, it seems that the synovial environment prevents bone formation otherwise induced by DABM in vascular tissue. Although tissue formed in articular defects supplemented with DABM is of cartilaginous differentiation, which is retained over time, it is of highly variable quality. Hence, the described approach has to be optimized before it can be applied for the purpose suggested.