An instrument for distracting and/or compressing adjacent vertebrae is described. The instrument includes a yoke, a first blade movably mounted to the yoke, a second blade mounted to the yoke and an adjustment system. The yoke, the first blade and the second blade form a working channel for the instrument. The adjustment system of the instrument is configured to adjust a spacing between the first blade and the second blade by selectively moving the first blade relative to the second blade to distract and/or compress adjacent vertebrae when the first blade and the second blade have been secured to the adjacent vertebrae. The instrument may include a depth stop system that limits an insertion depth of an associated tool inserted into the working channel of the instrument. The adjustable depth stop system includes a releasable catch for securing a stop element of the associated tool. A position of the releasable catch relative to the working channel is adjustable to vary the insertion depth of the associated tool.