Tip relaxation times detect early degeneration of joint structures, before morphological substance loss occurs, assess severity of matrix degeneration, monitor progression and help in orthopaedic treatment decisions. Additionally, T1 p mapping (either pre - or post-surgery) of different tissues may detect undesired development and failure and potentially predict the individual outcome after surgery. T1 p increases may precede and predict morphological pathologies.129 Therefore, the big advantage of knee repair outcome evaluation via T1 p is the early follow-up after surgery, while in standard radiographs, changes may only be detected after decades. This has such an importance, since patients tend to be young and healthy without any knee degeneration prior to injury. Joint function needs to be maintained before major osteoarthritic changes appear.

Compared to other techniques such as T2,T1p has been not been studied as intensively and needs further investigation in large longitudinal cohort studies post knee repair surgery. As other quantitative markers, it provides a specific measure of the composition of meniscus matrix, cartilage matrix and CR tissue. As an advantage over other quantitative imaging techniques such as dGEMRIC, T1p requires no contrast agent inject. T1p relaxation times seem to be more advantageous for cartilage evaluation, while T2 relaxation times were reported to be slightly superior to T1 p for meniscus evaluation. Despite this, both may provide complementary information on tissue composition. Although ceiling effects of T1 p may be smaller than in case of T2,6 these need to be considered when interpreting results.13’66’80 A lack of further increases of cartilage T1p in cases with severe cartilage degeneration may be due to a predominant loss of superficial cartilage and T1p may be “bottomed out” in terms of the biochemical metrics.38 For T1ho correlation with cartilage composition is somewhat understood, but it remains unclear, what exactly T1p detects in meniscus. This needs to be evaluated further in future studies, including histological evaluation.

Other potential applications of Tip after knee joint repair

Although first efforts have been made, there is still potential regarding other applications of T1 p after knee joint repair. Outcome after ACLR has been studied as described, but the different effects of ACLR techniques on cartilage composition have not been evaluated so far. In current studies, all ACL procedures were pooled together due to a small study cohort and groups containing patients with hamstring autografts, patellar tendon autografts or allografts. Outcomes of other ligament reconstruction surgeries could also be evaluated by T1p, including posterior cruciate ligament (PCL) reconstruction, medial patellofemoral ligament (MPFL) reconstruction, surgery at the patella ligament or at the posterolateral corner.

It appears that cartilage physiology and ultrastructure of repair tissue after different CR techniques can be determined and observed during the maturation process. Thus, similar to Mfx and OCT, other CR procedures including, chondroplasty, autologous chondrocyte implantation including its different generations and variants and osteochondral allograft transplantation have not been evaluated yet. T1p of CR tissue in correlation with clinical outcomes and other parameters such as MOCART scores or histological analyses are still outstanding. An additional helpful biofunctional feature would be evaluation of the reaction of repair tissue on loading.

Another application following knee repair procedures may include evaluation of outcome after meniscus repair. This application is challenging due to artifacts adjacent to the repair material. One patient with meniscus repair has been implied in the study of Li et al. ,20 and one patient in Haughom et al. ,17 but results were only presented in combination with other patients. Also bone surgeries, such as realignment by high tibial osteotomy, may be possible applications. Possible additional tissues on which T1p could successfully be applied are ligaments and BMELs. Until date, laminar analyzes have only been performed by assessing two cartilage layers instead of three due to a limited resolution. Further, no T1p laminar analyses have been performed in menisci after knee joint repair. Texture analysis parameters extracted from the gray-level co-occurrence matrix (GLCM) in T1p sequences77’91 may provide additional information in subjects post knee repair surgery.

Clinical application

Requirement of special software and calibration makes clinical application difficult. Stable sequences with calibration algorithms are desirable and an adequate calibration of different MR scanners is necessary to provide reproducible and precise data. For clinical application, a main issue is that standard reference values need to be defined for inter-person and interscanner reliability and comparability. Longer follow-up times of large cohorts post CR are needed to define an outcome predicting value and “normal” values for each time-point. The difficulty to define “normal” T1p remains from (i) differences between sequences and techniques, (ii) differences of T1 p between different compartments, sub-compartments, layers, weight-bearing, loading conditions, etc. and (iii) to interindividual differences of cartilage composition. However, these problems are less relevant during longitudinal observation of one individual or bilateral comparisons. Long and still varying segmentation processes need to be automated for two reasons: a long, time-consuming manual segmentation process and possible manual ROI measurement errors. Other clinical issues of T1 p are relatively long acquisition times and the high SAR due to high locking fields.

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