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This sounds like a typical boxer's fracture. Many times the interosseus mm connected to the medial border of the 5th MC is disrupted, thus inducing electrolyte imbalance and predisposing to cramp and inner range weakness. Though this mm is not a prime MCP flexor, it does assist this action and its stabilising reverse adductor action on the 5th MC is required for full powered grip strength. You could try heat, deep massage to the medial border of the 5th MC and its interosseus origin, progressive resistance training of such: i.e. start with pure outer range 4th phalanx abduction, then combined MCP abduction/flexion progressing to inner range as symptoms ease.