You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["AnatomicalEntity"].

Please return the output as a JSON object of the format: {"AnatomicalEntity": ["tumor cells", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

The effect of PCL elevation
Our hypothesis that the dynamics of the flexion gap depend on the orientation of the PCL was confirmed. Distraction of the knee with a low PCL elevation angle (i.e. a flat PCL) resulted in greater increase in gap height than distraction of a knee with a steep PCL. This result was to be expected; when the gap between the tibia and femur was increased, the increase in PCL tension resulted in the tibia pivoting around the femoral insertion of the PCL. Thus, the greatest increase in gap height will occur when the PCL is flat.
Taking this finding into consideration, the surgeon should be aware of the strong effect of PCL elevation on the relationship between gap height increase and tibial translation. A flat PCL will lead to a mean anterior tibial translation of 1.7 mm when the gap is distracted 1 mm. A knee with a steep PCL will even translate 2.3 mm on average with 1 mm of gap distraction. If the PCL were the only structure that was tensioned with distraction of the flexion gap, then at 100 N it would be oriented vertically. This is not the case, and therefore other structures such as the collateral ligaments must have restrained the translation. From an anatomical point of view, the collateral structures are the only structures that could have had a restraining function on vertical movements. For obvious reasons, in this in vivo study we could not test what would have happened to the orientation of the PCL when the collateral structures were resected.
In addition, the PCL elevation angle at 100 N influenced the increase in PCL elevation angle between 100 N and 200 N (DeltaPCLe). This seems logical: a flat PCL can be recruited more easily whereas knees with a steep PCL might already be tensioned at 100 N so that a further increase in PCL elevation angle would be hard to achieve. However, steep PCLs are not necessarily recruited more than flat PCLs. Furthermore, the situation during implantation of a TKR is quite unnatural: we investigated the PCL elevation after the tibia was cut.
{"AnatomicalEntity": ["tibia", "collateral ligaments", "PCL", "tibial", "structures", "collateral structures", "knees", "femoral", "knee", "anterior tibial", "femur", "PCLs"]}