A retractor assembly for tissue retraction at the site of an incision, the retractor assembly comprising: an arcuate frame member that extends around an arc of more than 270 degrees and less than 360 degrees; a mobile carriage including: a blade post holder attached to a blade post, wherein the blade post holder is configured to rotate about: a first axis of the mobile carriage, a second axis parallel to a tangent of the arcuate frame member, and a third axis that intersects the frame member at a location of the mobile carriage and a center corresponding to the arcuate frame member; whereby the mobile carriage allows the blade post to move with six degrees of freedom relative to the frame member; and a carriage locking mechanism configured to allow the blade post the six degrees of freedom of movement permitted by the mobile carriage relative to the frame member when in an unlocked position, and to prohibit movement relative to the frame member when in a locked position except translation along the groove; characterized in that the retractor assembly further comprises: a groove running along a proximal surface of the frame member, the groove including: a plurality of expanded portions along the proximal surface, and a plurality of narrower portions along the proximal surface; wherein the mobile carriage is configured by means of a track engagement feature to initially engage with at least one of the plurality of expanded portions of the groove and, subsequent to the initial engagement, engage with at least one of the plurality of narrower portions of the groove to allow the carriage to translate along the groove; and wherein the track engagement feature fits into the groove such that the track engagement feature resists leaving the groove while the track engagement feature is disposed within one of the plurality of narrower portions and permits leaving the groove while the track engagement feature is disposed within one of the plurality of expanded portions, wherein the track engagement feature constrains the carriage from yawing relative to the orientation of the groove. an arcuate frame member that extends around an arc of more than 270 degrees and less than 360 degrees; a mobile carriage including: a blade post holder attached to a blade post, wherein the blade post holder is configured to rotate about: a first axis of the mobile carriage, a second axis parallel to a tangent of the arcuate frame member, and a third axis that intersects the frame member at a location of the mobile carriage and a center corresponding to the arcuate frame member; whereby the mobile carriage allows the blade post to move with six degrees of freedom relative to the frame member; and a carriage locking mechanism configured to allow the blade post the six degrees of freedom of movement permitted by the mobile carriage relative to the frame member when in an unlocked position, and to prohibit movement relative to the frame member when in a locked position except translation along the groove; a blade post holder attached to a blade post, wherein the blade post holder is configured to rotate about: a first axis of the mobile carriage, a second axis parallel to a tangent of the arcuate frame member, and a third axis that intersects the frame member at a location of the mobile carriage and a center corresponding to the arcuate frame member; whereby the mobile carriage allows the blade post to move with six degrees of freedom relative to the frame member; and a carriage locking mechanism configured to allow the blade post the six degrees of freedom of movement permitted by the mobile carriage relative to the frame member when in an unlocked position, and to prohibit movement relative to the frame member when in a locked position except translation along the groove; a first axis of the mobile carriage, a second axis parallel to a tangent of the arcuate frame member, and a third axis that intersects the frame member at a location of the mobile carriage and a center corresponding to the arcuate frame member; whereby the mobile carriage allows the blade post to move with six degrees of freedom relative to the frame member; and a carriage locking mechanism configured to allow the blade post the six degrees of freedom of movement permitted by the mobile carriage relative to the frame member when in an unlocked position, and to prohibit movement relative to the frame member when in a locked position except translation along the groove; a plurality of expanded portions along the proximal surface, and a plurality of narrower portions along the proximal surface; wherein the mobile carriage is configured by means of a track engagement feature to initially engage with at least one of the plurality of expanded portions of the groove and, subsequent to the initial engagement, engage with at least one of the plurality of narrower portions of the groove to allow the carriage to translate along the groove; and wherein the track engagement feature fits into the groove such that the track engagement feature resists leaving the groove while the track engagement feature is disposed within one of the plurality of narrower portions and permits leaving the groove while the track engagement feature is disposed within one of the plurality of expanded portions, wherein the track engagement feature constrains the carriage from yawing relative to the orientation of the groove.